Yes it is expensive but my model year van didn't have the factory brake control module sockets so it would have meant splicing in a wired brake controller. Like you, I figured the dollars saved on labor using the DIY wireless unit would probably equal it all out.
The 4 way to 7 way adapter I used had a plug to insert the 4 way connector that was already on the van. I taped off two wires - the brake wire and the backup light/surge brake wire. I used the two other wires - the 12V+ power wire and the 12V- ground. I put a 30A circuit breaker on the 12V+.
The 4 way to 7 way adapter I used had a backup beeper but I disabled it.
some info: http://www.classbforum.com/forums/f8/wiring-for-trailer-3979.html
Originally I installed the connectors for a cargo box and other accessories but have since been towing a travel trailer with the van.
I don't see any reason to not make the 12V+ connection where the house battery connects to the converter as long as that power source can't be disabled or interrupted by a battery disconnect switch etc.
The Prodigy RF main unit mounts to the trailer frame and the wireless unit plugs into a 12V accessory outlet on the van dash.