the AI won't sell me any planets because they only have a couple apiece
Near the end, they want about ~6,000,000BC per planet. Sometimes they'll ask for as much as 10-11 rather than 6.
I ignore the penalty, for the most part. It does hurt your score somewhat, but it's not as huge of a penalty as everyone thinks it is-it's only 20%, and applied as a racial penalty. So if you for instance have +300% econ from techs and resources (not outside of the realm of possibility, particularly in TA), you're only getting +280% out of it. This results in you having 95% of the gross income you would otherwise have, and due to the fact that everything in the game gives you less and less score as you increase the value thereof (see in particular Mumble's infinite posts about how 2x military rating does not give as much military score as x military rating, same applies for economy, etc), it is not a farfetched idea to imagine that in this kind of scenario your economy score would only be approximately 5% less as well, although I doubt anyone has specifically run the numbers.
The aside to that of course is that if you have that much cash on hand and are not spending it, the player in the same situation who is spending it is getting something scorewise in return for their investment-but this doesn't necessarily mean you need to spend below the threshold, as this would be true comparing even someone who spent down to 30000BC every turn and someone who just let the cash roll in every turn.
But if you're a true scoremonger, you'll want every point you can get.
Considering late game ships can get up to ~4000BC which is ~55000BC rush buy cost, you should almost certainly be able to spend yourself under the penalty threshold each and every week.