He's more than just cheap. 'Practically free' creates the value in this context. I don't much care for Burger King, but if the app gives me a special mobile deal some week enabling me to get a Whopper for 3 cents for a limited time, I'll probably drop by and use it before the coupon expires. 30 grams of protein for free? Sure, why not.
As for Moreau, I don't see a pass-catcher on his tape. He's slow off the line, he doesn't separate from even average linebackers in coverage, he labors into and out of his breaks, and he doesn't attack at the catch-point. Waller isn't the long-term answer at TE in Oakland, but I think the only snaps Foster's likely to take from him will be on blocking downs.