I believe orbiter addons should have the right to accept donations but not charge a set price. I think i speak for a lot of people when I say addon devs enjoy doing it thats why they do it, but a donation wouldnt hurt once and a while :P
I just built a pretty amazing computer for under 500$ i had a case already (30-70$) mid range mother board(forgot how much it cost) intel i5 quad core(70$ my dad works for intel i got a discount) ATI Radeon 4650 graphics( 99$ future shop had a amazing deal and 4 gigs of ram ddr3( 60$) i can run...
i could be wrong but to my knowledge orbiter likes to use a bit of ram upgrading to 2 gigs should make a noticable difference its fairly cheep also (around 20$)