I would go back to him and complain - tell him he said 3-4 hours (would help if you found out his hourly rate first somehow) and you expected to be charged for 3-4 hours only.
On the machine - the smaller machines cost about $500-1000 complete - the larger ones for big systems and very fast recovery cot about $3-5,000. Obviously the $250 freon recovery charge includes presumably both labor and tools - so lets say $50 range labor leaves about $200 on the machine charge - so if a small unit (hand-carried, about size of a portable defibrilator or mid-size microwave) then he intends to pay it off in 3-5 uses ? Even if a big machine (about the size of a substantial welder or close to dishwasher size, on a cart) then 15-25 uses to pay off - that is unreasonable - even that would be probably a couple of weeks to payoff for a machine that should last at least a couple of years.
Sounds like you have basis to argue - but besure to get signed invoice showing PAID IN FULL before you are done, so he can't come back and slap a lien on you without it being fraud.
Lesson learned - NEVER proceed without at least a written estimate (which in most states cannot be exceeded without PRIOR approval), and preferably a firm bid.
BTW - went to a couple of contractor and consumer blogs on this item - consensus of recent costs for replacing conventional evaporator, complete WITH coil, generally in the $800-1100 range, so that makes his charge WITHOUT evaporator look to be about 100% high by comparison, unless you are in a place with crazy high prices like SFO/Silicon Valley or highrise or upscale NYC or Boston.