It depends if you're willing to shop on ebay or not. If it all has to be retail, then you indeed can get away with spending the least on Grey Knights. Space Wolves can also be pretty cheap too, especially if you use a lot of wolf guard and characters.
If you're willing to shop on ebay, you can get some very good deals on the following, thanks to the overabundance of Assault on Black Reach sets put on there:
Space Marine Captains
Space Marine Infantry
Space Marine Terminators
Space Marine Dreadnoughts
Ork Slugga Boyz
You can get either dirt cheap, as long as you don't care that they all have the same pose, and your special weapon/gun choices are already set, unless you're ready to convert them yourself.
If you care a lot about competitiveness, the marines are probably the better choice. Orks are good, but you generally need to support them with plenty of vehicles.
The first casualty of war is neither innocence nor truth.
The first casualty of war is always subtlety.
Last edited by mynameisgrax; 11-18-11 at 03:55 PM.