I'm wondering what site might be offering the cheapest domain registrations for this week? I know sometimes different sites have specials and what not.
Do a search for coupons here, it depends on which extension, if it's a new one, etc.
If it's for a .info name, try this GD trick where you can get a .info domain for $1.99 (without coupon code or minimum purchase, etc)... it's in French, but the screenshots should pretty much explain the process.