Which head piece would be the cheapest?
One way to do this on Bricklink is to run an advanced search over the "Minifig, Head" category. Here's an example showing all yellow parts in this category with a minimum lot size of 200 sorted by price, lowest first.

This shows you the cheapest stores to order 200 of the same yellow head from. If you want a little variety, I'm sure those sellers have other heads available as well.