Considering there wouldn't have been an unlimited number, I would say decently rare.
What makes it unique? Does it have a stamping or something that says preproduction?
Hard to say... It's worth whatever a collector is willing to pay. Since there is nothing to base it off of and the Trx4m isn't exactly a polarizing model a collector would be interested in, its likely not worth much more than a conversation starter.
I'd hold onto it and just enjoy the fact you're one of the only people to own it(I also have a RC hoarding problem, so take that as you will)... While its likely not going to contribute to your retirement anytime soon, it *could* be worth *something* in the future.