It is pretty much like putting together a Lego set, except the directions are much less details and you have to improvise a lot. I don't think it is hard, just watch a few videos on how to do it, they are all built the same way except some parts are installed different ways.
It really just depends what your trying to get out of it, if this computer is just going to be used to store information, and check emails and do daily things then you really don't need to spend more than 650 dollars to get something that would work really well. You really start saving money when the price of parts goes near the $1000 mark, where companies usually charge around 200 bucks for putting it together.
My advice, look at some premade desktops from Dell or IBM and try to find something in your price range.