i cadded a set of mechanums that i calculated would cost less than 50 dollars a piece, but they require a 5 axis mill, and some casting work, i could dig out the cad files if you like. if you want to know all the math involved in making them, here we go:
the most important thing to do is to understand that the profile of the roller is a partial ellipse, where the short dimension is the diameter of the wheel, and the long dimension is the diameter of the wheel divided by the sine of whatever angle the rollers are at (usually 45) after that bit of math there is really not much to designing fully custom mechanums.
http://chiefdelphi.com/media/photos/35738
http://chiefdelphi.com/media/photos/35738
here are the renders of the mechanum wheels that i did, the blue one should cost around 50 dollars, (depending on what casting compound is used for the rollers.