I'm brainstorming on the design and will work on a concept version for myself over the next 6-12 months. If that works well, I'll build more after our new house and shop is built.
It would be all new parts, probably be a single cylinder, single stage with a 1/2hp-1hp 120v motor using a shop air compressor to provide 120-140psi to the piston. 3000psi output would be the goal. It could be used for filling reservoirs or smaller tanks. The main problem I'm having now is keeping the heat to a tolerable level for the Teflon piston seals.
If you think you're going to get a motorized HPA compressor for a few hundred dollars, think again. A basic hand pump runs $180-$300. If it works out well, design-wise, and has good durability, I think $500-$1000 would be the target price. A SCUBA tank cost a few hundred dollars plus the cost of the fills plus the cost/time to drive to a dive shop. Also, if you had an AR buddy or group of AR buddies, you could split the cost.