A lot of people want to use a particular hub, or rims. For example, you can get a Powertap/DT Swiss hub for less than $200, but I don't think it's available for 36 holes. ($170 in 20 hole.)

Given that numbers of spokes don't ensure a strong or quality wheel, and that how many spokes is less important than what spokes and other factors, finding a good wheel builder is probably more important than having 36 or 72 or whatever spokes.