so my question now is how much can a shop true up a wheel through spoke adjustment?
Seems like the combination of replacing the axle, skewer + truing cost the same as an cheap replacement wheel.
Actually, the economics are worse than you describe. I find good MTBs wheels cheap all the time, bought a set (attached to a bike) for $5 at a garage sale Saturday, included wheels, tires, cassette, tubes, etc........ It is not unusual to find used MTB wheels for $5 to $10 each. Since you are starting with a low cost bike, I would hunt for a donor bike or a cheap set of used wheels.