My least expensive one I like is the Proraso Pro Boar Brush with silver handle. It's an Omega Pro 48 with a shorter loft, which I prefer. About $10 on the Bay.
I have two cheap favorites, a boar and synthetic. Favorite boar is Omega 10065 - $11 and comes with a stand and omega shaving cream. Favorite synthetic is Stirling lil’ brudder for $10
All my brushes are pretty cheap. The most expensive brush I have is a Whipped Dog Silvertip with a custom ceramic handle (I don't think they offer those anymore), and while it's a looker, it's my least favorite to use (I prefer boars). Cost me $35.
I just ordered an Omega 10049 for $9. Time will tell, but it could be the winner.