Hmmm, so it seems there's a bump up between $3k C/I and $4k C/I, because last year I put in around $3k C/I and my offers are 3 nights with $50 in FP and $50 in MP, plus a $25 in F&B and BOGO show ticket. Although it could also be that I was playing VP. . .
VP play definitely earns you less mail offers than slot. I am also guessing from the $50 MP offer that you played some table games too. If you play both table and machines at the same location, your offers are usually divided like this and the total is lower than if you had just played one or the other. To maximize offers, you should have certain stores where you play tables and certain stores where you play machines, but of course that is hard to do if you are from out of town and staying at the property, as naturally you want to play at that property.
There are sweet spots for every location, as far as amount of coin-in and such. It's not proportional. Maybe $4K coin in earns you x amount of free play, and only a bit more say, $5K coin in will earn twice that. The trick is to find the sweet spot, maximizing return for minimum amount of play. This is mostly trial and error, with some info sharing also helpful. But just when you think you have figured out the system, they seem to tighten up or lossen up. For the last year, all of Vegas seems to be going in the same direction. :(