liquid nitrogen costs about $2 per gallon according to this site:
you also have to make a heatsink that you can contain the liquid nitrogen in. but it must have an air hole, so the hotter nitrogen that turns into gas can escape.
but if you use liquid nitrogen, don't expect hours of gameplay using it unless you can keep refilling the LN.
in fact, LN is really only good for running maybe a few benchmarks at a time.
if you wanted to use LN 24/7, you'd have to have lots of it shipped to your house every day, and have a system that keeps topping up the LN. and that just isn't practical.
on the other hand, vapour phase change cooling, can be bought as a kit (look for Vapochill products) but is really expensive. I think the minimum for a vapochill kit is about AU$1000 (or probabbly about US$700 or so)
you could also build a vapour phase change cooler. however, the materials still cost a fair bit of money. you have to get the pipes, insulation, compressor, condensor, and the cooling fluid.
and you'd also have to know how to weld reasonably well, and have an understanding of how it works.
for the average person, the most you'd do is buy a vapochill cooler kit. all you have to do is set it up. you don't have to make the cooler itself.
but that's still only if you're prepared to pay the hefty price for it.