basically, the main differences are, besides the obvious size is the resolution.
The 7" is native 800x480, the 8" is 800x600. Both use a 6 bit TTL panel, both have stock brightness in the 350-400nit neighborhood (depending on who's website/spec sheets you look at) and both have the same cheap touch screen.
The average price difference between the 2 is about 20.00. This would be through regular online retailers or ebay sellers.
I guess mp3car is keeping the price difference comparable to that.
The actual cost to upgrade either one shoud be close if not the same.
shoot me a pm with your email and I will give you a little more info.