Given how packed my local Lego store has been since 'The Lego Movie' came out this really isn't a surprise. Also since said customers may not be that interested in this figure, I imagine a large volume of the promotional figures will wind up on Ebay for a reasonable cost.
Also to provide some context, Lego ran the same promotion for the Hulk figures a few years back. I believe it was for a lower purchase price ($50 I believe) but I know for my local store they definitely ran out before the month was up.
And now I also have to remember not to buy my Star Wars allotment just yet as I may be hard pressed to get enough OT sets to make the $75 promotion. (Unless of course they offer a Prequel figure, in which mine will wind up on Ebay for the rest of you procrastinators)
It would be nice if one month they could have multiple figures and let you choose. Perhaps that would add complexity to the promotion, but at least people making a random purchase would be more likely to get a figure they cared about. I'm an old school comic guy, so I was very please to get the MM figure, but I don't think my kids would know who he is. (Unlike Green Lantern, which we're all STILL waiting for.. uggh).