NPD is $10,000 per report per month. If they find out you'd ever transmit the data elsewhere, you'd probably be in breach of their NDA or other licensing agreements.
Brett left VGC and with it, any hope of adding proper methodology.
The Patreon idea isn't a bad idea. If you need help on how to calculate sales for games/DLC, just give me a PM and I can tell you how to do it. I worked with Brett and a few others professionally for a few years on digital game tracking (FADE). I don't think the concept has changed much since I stopped doing it professionally. But you WILL need someone that's very, VERY fluent with data scrapers and at least one high-end ($200+/mo) server, for starters.
Back from the dead, I'm afraid.