I have several, and like the LG WCP-300 the best, I think was around $30.
http://www.lg.com/us/cell-phone-acce...s-charging-pad

I have two like what catflyer posted, and don't think they are strictly complaint to Qi. Once my Nexus 4 reaches full charge, the charging stops. There is no periodic top off and if I leave phone on the charger for many hours, I am basically guaranteed a not fully charged battery. The quality control is also suspect. One of chargers only works with one specific receiver. The price is pretty good though (I paid less than $10 for each, shipped from South East Asia).