hey, im a bass player...and usually for somethin like this i would kno the answer but this is an acoustic guitar my moms friend just sold me for $50.....not that great of a guitar but hell its a guitar for $50

so heres my problem
the neck is to thick for my liking, and i like modding guitars so i figure id see if there was a way to sand or shave or cut or somethin it so its a bit thinner with out destroying the neck totally
and by thick i mean like between the back of the neck and the freboard

so can i do this?

and how is it done
shaving and reshaping the neck is a pretty tricky process, if you don't even know where to start you should really have a pro do it.
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I say go for it. Like you said, it's only 50 bucks, and if you f it up, you aren't out much. You're gonna need a couple of specialized tools tho, unless you plan on just using sandpaper, but that'd take forever. A shaping rasp followed by sandpaper would do the trick. The rasp will take off a fair amount of wood with each pass, but will allow you to keep the contour of the wood at the same time. Once you get it to the thickness you want, sand it smooth and refinish it. Could be done in a day if you have everything on hand to begin with. Another tool that will remove a lot of wood fast is a plane, but I don't recommend it as they tend to gouge a lot if the surface to be planed isn't flat.