If you plan ahead and have some sort of idea what you're doing (LOTS of reading) then it shouldn't overrun in cost too badly. My first conversion was a bit costly but the second one came in right on track. The trick is to thoroughly count the cost BEFORE you start buying.

For example, find out what kind of chainline your prospective rear wheel will provide. Its going to be 42mm for a road spaced hub, and 52mm for a mountain bike spaced hub (135mm). So, measure the chainline of your existing chainrings. The middle (on a triple) or the inner (on a double) will probably be 42mm or darn close to it and the outer ring is usually 52mm. If its too far off then you can PLAN on the cost of a new bottom bracket that is appropriately shorter or longer than your current one. That way you don't have to fool with chainline when the project is done. You plan it BEFORE you buy anything. The chainline is the biggest variable in a fixed gear conversion, and the "what ifs" and surprise costs can be eliminated by doing some measuring and planning. Have I used the word "planning" enough? LOL

The other things to watch out for are the same things you'd look out for when building any bike. How is the headset? Tires? Brakes? Assess each of them and plan accordingly. There's no reason a person couldn't order everything all at once and and put it all together in an afternoon or less. My second conversion went this way. It was done in under an hour and the chainline was perfect and I didn't have to make any unexpected trips to the LBS.

So, it CAN be done, and it does NOT have to be expensive at all.