**SPOILERS PAST S03 7TH EPISODE?**
Jesse made mention of this in an episode too when he started to skim meth to sell himself after becoming Walts assistant. No the math doesn't "add up" but as other said its a risk/reward thing on Gus' part. He's providing the location, equipment, and distribution (great scene at the beginning of one episode showing in commercial-esque fashion how he uses Los Pollos as a transportation ring) besides Walt can't even spend the half million he has without raising suspicion, Gus knows this and problaby used it as leverage to come up with that "low" figure of 3 million. Walt isn't a greedy man he just wanted financial security for his family.
Also, when Gus bought his 38 pounds they weren't "in business" together and he had to pay street value so he could test the market for it from his end, at the time Gus only heard it was excellent and did not know of how well it would sell. I hope season 4 expands the horizons from this POV some more, the story can literally go anywhere right now. There's still Tuco's uncle in the wheelchair and the cartel, there's Hank on the road to recovery who will probably have even more of an interest in finding Hisenburg whether he's DEA again or not, there's the fact Walt is bonding with Skylar again and they may reconcile, and oh yeah.. Walt and/or Jesse might be killed! Will have to wait and see, unfortunately.
