Okay, so the reasonable assumption would be that the balances have a log-normal distribution. Just spot-checking by eye, these percentages are a pretty good fit to a survival function with mu = 299M and sigma = 1310M. Plugging those in to the relevant formulas, you get:
Median: 50% of Inara profiles have a balance above/below 299M
Quartiles: 25% of profiles are below 124M, 25% of profiles are above 725M
The 90th percentile balance is 1.61 billion, and the mean balance is 707M.
Of course, the big question is how representative Inara profiles are of the player base at large. One tends to suspect that Inara users skew more active, and thus are probably richer than the typical player. But by how much, who knows!
Certainly there's going to be a big clump of people who purchased, played for a bit, decided it wasn't for them, and are sitting around with a Sidewinder somewhere - and therefore certainly never joined Inara. On the other hand, they're probably not relevant to any "how much should things cost" either.
Excluding them, those numbers certainly feel about right - anyone with over a billion in cash already has nothing to spend it on, so probably already has every ship they want fully outfitted. Net assets would probably be a much wider distribution, of course, but probably still close to log-normal.
(Log-normal seems likely to be a good approximation - CG thresholds tend to look similar, with the top 10% players often contributing about half the total volume - and squadron sizes I think would also fit a log-normal curve reasonably well)
I'd prefer to know what the mode is
Almost certainly 100 credits, if you take a strict definition, because lots of new accounts will have that, and with probably under 4 million total accounts and a 90th percentile of a billion, any other number of credits is highly unlikely to have more than one player, and almost certainly no more than two or three.
If you group into bands then a log-normal distribution would probably give a modal band that wasn't the lowest (as long as the bands weren't too wide, and maybe excluding people with exactly 100 credits entirely), but it would still be pretty low.