Yep, have pretty much figured that out re. sensors, leaving then the questions of whether doubling the resolution from a D90 to D7100 is much use in the same sensor, and whether bigger camera with more useful layout is worth the hassle of carrying it round:-)
As to how many bucks then they are limited but would be happy to shell out on a second hand D7000, or maybe even D7100 if could see it was a worthwhile investment...
Nobody said it was going to be easy:-)