Realistic the 7px is the same price if not cheaper than the m17. The cheapest I found the m17 and 7px is on rcmart.com. The m17 is $520 shipped and 7px is $560 shipped with 2 receivers. So price is about even.
Futaba receivers are cheaper than the sanwa. With Futaba you can run the non telemetry r204gfe receivers for $47 shipped or the high end sr tememtry r334sbs receivers for $60. Sanwa receivers are close to 80-$100.

and yes you can charge the 7px by plugging it in to a wall outlet but this can only be done if your run a 6v AA nimh battery pack when can be bought for $15. Most people don’t do this because most people run a 2 cell life battery. I run the 2000mah turnigy battery that cost $25.