For registrars, the margins are basically non-existant, low cost + high volume is where they're all already at.

For .com domains, Verisign charge all registrars $7.34 per registration (going up to $7.85 in January 2012), so when you consider GoDaddy charge $11.99 (less with various offers), it's not a big margin business.

There's always a market for lower cost, but you're not going to beat the prices of the highest volume registrar in the world.