Which head piece would be the cheapest? I would like to fill a vase with LEGO heads to decorate my living room so I am looking for the best option to buy bulk heads. 

I have been looking at Bricklink but I can't find a good way to tell. I want 200 (not sure about the number for the moment) yellow heads, don't mind what the face painted looks like but I want to find the cheapest option.

Has anybody got any clue on the best way to do that?
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.