Great idea, but it won't work. Your math is a LONG way off. First, it doesn't matter how many people in the US have a detector in the closet. 90% of them are only used a couple of times in their own front yard and then sold or collect dust for decades. We have a true core only numbering in the thousands of REAL detectorists and most of them don't ever get on a forum or any kind of online detecting site.
Lets say we could get 100,000 detectorists to join for $20 a year to fight for their rights. $2 million is nothing. You need office space, employees, webhosting, advertising, airfare, car rentals, hotels, and a HUGE budget for "greasing the wheels" of the politicians.
$2M would probably get your wanted results for a single state, but when you start talking about nationwide your costs increase exponentially.