The parts are not expensive alone. Once you add in the tubing, fittings, coolant additives, etc. it adds up very quickly. If you go with budget parts, you'll also be betting the entire computer on whether or not the fittings leak.
An air cooler will be far more reliable at that price point, and the rest of the budget can go towards acoustic treatments, and perhaps some clever duct work to get the most cooling with the least noise in a desk build.
Can you post some links to the parts you have in mind? You could have come across an awesome deal, but if you need more parts than you expect, it's best to know sooner rather than later.