None. Silvers (Osteoglossum bicirrhosum) are the cheapest and most common. Price will also depend on how you get them, either through your LFS or online/shipped.
I'm partial to the jardini (Scleropages jardini), going to be the fourth and last fish I introduce to my tank.
A helpful
link I found in my research, written by David Haltinner of AquaWorldMagazine.