Varies a lot.
You can see what people are selling for at sythe here:
http://www.sythe.org/forumdisplay.php?f=268Do note that like 80% of the threads/people at Sythe are scammers though :p
So they list low prices to attract customers etc.
Because of the recent GP dupe/other glitches price of GP has fallen due to inflation.
Here's my selling thread at sythe:
http://www.sythe.org/showthread.php?t=1422948And my website:
http://memberize.me/I've sold over 300 upgrades.
Price for 30 day from me is: ~20M though.
You can try your luck at Sythe for a lower price, be SUPER CAREFUL, and ideally
ONLY accept GameCard upgrades.
Other upgrades can be 'charged back' leaving your account locked, and then you have to pay $20 to unlock it.