I know many DDI subscribers. None of them has ever used the WotC forums (or the ENWorld forums, or the RPGnet forums, etc. Most gamers don't go in for the cesspool of online forums). I have a very hard time believing that the 45,000 figure represents more than half of actual users.I think the number is more to 75-90% of total customers. Whoever is invested enough in D&D to find the WotC website and subscribe to DDI would surely log into his WotC account (which needs to be created to subscribe to DDI in the first place) at least once. And after that he's automatically part of the DDI group.
I seriously doubt they're even close to have a 5-digit number of subscribers
My guess would be that there are between two and five times as many total subscribers, which, going by the average subscription price, would put their yearly gross somewhere between $8.59 and $21.47 million. Even at the low end, that's a lot of cheese.