Just from personal experience - go with the highest quality/weight pad available. But if they have a "stain guard" one that is even more expensive I fail to see the value in that.
A plush pad makes even the cheapest carpet feel pretty good under your foot.
As far as getting it on the cheap, hit up a carpet outlet and see if they have a leftover roll on discount that will fit your room. If they are smaller areas, you can get by pretty cheap. We paid $1.22 a sq/ft for the carpet in our basement that a normal retail place would charge $4.00+ for.