i want to record my guitar at home, but just my guitar
now i see all this interfaces raging from all kinds of prices
but i really just need one input for my guitar

Is there a quality difference from interface to interface? or really what makes the price different is the number of inputs?

I dont wanna ask the shop staff because last time they tried getting me to but the most expensive on.
the tascam TNT pack is really good, 100 bucks for software and an interface, and a mic too, which can come in handy, whether you wanna mic your amp, or do vocals (but you can plug the guitar right into the interface). connects by USB and works on both pc and mac.
Fender Hot Rod Deville 2x12
Custom Built Guitar (made it myself)
PRS SE Soapbar II Maple
Fender Stratacoustic (Stolen! )
Digitech RP200
Boss MT-2
Roland Microcube
I like my stuff!
There isn't as much variance in i/o boxes as there is in like.. microphones. I'd say just get one of the mid-level m-audio boxes. If you spend around 200 bucks on an interface made by a company like m-audio, alesis, or digi, you'll probably be happy with what you get. As long as you get something that's low noise and has a good sampling rate (which is pretty much any of them these days) then you're good.

You really only get into spending a lot of money on computer recording interfaces when you want a lot of inputs or really high quality mic preamps.