So I'm a geek because I did a bunch of calculating. Thought I'd share with the rest of the gang, maybe it will be of some use to someone somewhere.
First Column: level of planet
Second Column: colonists per tech level
Third Column: Max colos per tech level
Fourth Column: Base colos.
Fifth Column: Max colos per planet at that level (tech + base)
Sixth Column: 90% of max colos, to try to prevent revolt.
Seventh Column: The amount of credits a level will hold.
Eight Column: The amount it will cost to upgrade to the next level (I went up by five)
Ninth Column: The total amount it cost you to upgrade to that level (I started at level 150)
So... A level 350 planet would cost you 94 Trillion total to upgrade, while it can hold 74.3 Trillion credits.
It wouldn't let me attach an Excel file, so here's a webpage to look at:
http://dutchintexas.freewebpages.org/Planet%20calcs.htm