Wealth and opportunities in the Solarian League
The Sol system, like many systems in the Solarian League, has a lot more income stratification than someplace like Manticore -- or, for that matter, the Republic of Haven -- and there are additional factors which helped to account for the differential in per capita income.
For one thing, the Old League's core worlds tend to have much, much larger and denser populations, and the Sol system (in particular) attracts a lot of refugees and escapees from the less prosperous systems of the League and those under Frontier Security mandate. Once they arrive, they contribute to large pockets of undocumented, "under the radar" ghettoized poverty (a classic "underclass" which gets the sort of shrift underclasses traditionally get). The majority of Solarians frequently don't even realize that those pockets of poverty exist, and don't have any very clear notion of just how bad things are for the people living/trapped in them. They assume that there is a sufficient "safety net" to keep everyone afloat in relative comfort, if not precisely affluence, without realizing just how great a gap there is between what they think of as "relative comfort" and what the safety net in question actually supplies. Moreover, they don't realize how many of the system's poor are completely undocumented and, as such, largely ignored by the governmental support agencies. Nor do they realize that the Sol system has a policy of deporting non-citizens, which is one reason that so many of the people hiding in the various ghettos are undocumented in the first place. A lot of this is the result of the bureaucratization of government throughout the Solarian League. Bear in mind that the bureaucracies in question have had literally dozens of centuries to be created, balloon into nightmares which laugh at the very thought of meaningful reform, and be taken out of the electoral process because they've been so thoroughly institutionalized. The people actually working within the various bureaucracies may have an excellent idea of what's going on, but they (and their clients) are thoroughly insulated from contact with the mainstream societies around them. Some -- indeed, many -- of the bureaucrats are genuinely attempting to do their very best for their charges, but that degree of insulation and isolation means that they will probably never receive the degree of public support and demands for change that the repair of the system would really require.
What all this really means is that the typical middle-class Solarian doesn't realize there is a significant population living in poverty ("out of sight, out of mind"), that the "safety net" system which is supposed to be serving that population's needs is itself desperately in need of restructuring, or that there is a significant "gray" economy dealing with that population's needs and never contributing to the tax base (not directly, at least) or being accounted when things like system gross product are calculated.
In addition to these relatively large pockets of extremely poor people, the gap between the income of the very wealthy and the average Solarian is stupendous. By the standards of the majority of the human race, even the poorest Solarian lives a wealthy lifestyle, but all things are relative. Klaus Hauptman, despite his enormous wealth (and make no mistake about it, his personal wealth is absolutely staggering on almost any conceivable scale), probably wouldn't even make it into the Solarian Fortune 500, if for no other reason that the people who do make it into the Solarian Fortune 500 tend to have been there for generation upon generation. For them, the mere thought of worrying about how much something -- anything -- costs would be ludicrous, and they live accordingly, for all intents and purposes completely insulated by power, wealth, and prestige from all of the unpleasant realities the rest of the human race faces. This is acceptable because despite the enormity of the gap, the average Solarian income and lifestyle is very comfortable and affluent. The absolute inequality may be huge, but there is very little feeling on the part of the vast majority of Solarians (as in inhabitants of the Sol system) that they are deprived or living hand-to-mouth existences and, for the most part, the very wealthy are as far beyond the average Solarian's radar screen as the extremely poor.
Another factor in the lower Solarian per capita income (and wealth generation) is that the Sol system is much more prepared to accept inefficiencies than Manticore is. The Sol system is so huge, and so wealthy, and has so many people living in it, that it is essentially a "satisfied economy." I'm not saying that it's stagnant, or that there's no capital investment, by any means. What I am saying, however, is that the Sol system does not place the same premium on innovation that the Manticoran system does. There is a basic mindset that "we're doing just fine" and "if it ain't broke, don't fix it" which precludes the Sol system from being as "hungry" when it comes to innovation and basic research as Manticore, not least because the Sol system has such an enormous internal economy. Despite the fact that interstellar shipping in an impeller drive, hyper-drive equipped economy is incredibly cheap, it's still cheaper to produce goods locally, so even though the League enshrines the principle of "free trade," there are effective trade barriers simply because of distance and time in transit which tends to buttress (or at least subsidize) the inefficiencies hidden within the Sol system's economy. And the Solarian view (which is probably eminently sensible in a safe and secure society at the apex of the galactic political pyramid) is that the function of the economy ought to be to meet the needs and security -- and satisfaction -- of the people living in it. Solarian moral crusaders may be relatively rare, but so are Solarian ruthless robber barons, and the Sol system's labor force is more concerned with making sure that it has the sort of health care and financial security it wants, coupled with the least onerous working conditions possible, that it is with worrying about whether or not the Sol system is the leading economic powerhouse of the universe.
What all this means is that there is a vast potential for upgrading the Sol system's economy if changing economic or other factors should make that desirable, but at the moment, the Sol system is substantially less efficient at generating income (or funds for capital investment) even from the same sort of income sources (see my comments below) than the Star Kingdom of Manticore has traditionally been.