These kind of things ain't cheap.
Azden makes what is as far as I know, the cheapest of the cheap: $250.
Their cheapie model is also VHF, which doesn't have the range or interference rejection capabilities of a UHF system, which is , surprise, more expensive.
I would get a
wired lapel mic (a "lavalier") and run it down the actor's leg. Most of the time this will be out of frame, or can be easily hidden. The only time that won't work is when the actor is really far away. Here I would dub in the dialogue as suggested above. Dubbing works
so much better when it's hard to see the actor's lips moving.
Wired lavaliers can be had for as cheap as $20, though I would spend as much as I could.
HTH