I pay laborers who operate a chainsaw $12-15/hr. I'd pay a good machine operator who is preventative on maintenance, works without a lot of supervision, and isn't on the phone or smoking all the time, at least $15-$18 maybe $20/hr depending on how much they are making you. Maybe start at $15/hr, guaranteed hours, and help with fuel when he has to meet you at a distant job site in his own vehicle.
I'm just guessing here since TX wages are less than other parts of the country due to the influx of cheap labor. Down here there are some down-right awful, unskilled operators driving all types of machines. My favorite is the backhoe guy in a rented backhoe on a major highway project trying to dig with the hoe and no stabilizers down! he wasn't worth $10.hr.