Is this roto-tiller repair bill reasonable? And what are our options?

We have a big Troy-Bilt tiller (I think it's the biggest one; costs $2800 new; might be the next one down, $1900 new). Had the engine rebuilt 3-4 years ago for $250. This year, it wouldn't run. When I dropped it off for repair, he gave me the impression it would be a simple and cheap repair. I asked for a wag on the price--maybe $50-100? He didn't say, but said he'd call me if it was expensive. I just found out he replaced the carbeurator and wants $425. I did NOT authorize that kind of repair!
Not having authorized the repair, I can demand he take the carb back and give me my tiller back, right? Of course, I don't know what we'd do with it--I mean, a tiller that won't run isn't worth anything at all (I do not need a space-taker in the garage!), but still. . .

I was really looking forward to having a garden this year, but realistically speaking, we don't get that much out of it. I'm not sure it's worth another $425.

Any suggestions? We can't do the repair ourselves

He said it was ruined by the ethanol separating out of the gasoline and sitting in the carb.

Since he didn't call and get your authorization for the $425 repair, you're prolly within your rights of requesting your tiller and old carb be returned less a nominal troubleshooting charge.
Prob is you might end up in jp court before you get your stuff.

Just guessing now but most carb problems respond quite nicely to a good cleaning.
Seldom have I had to replace a carb except for extraordinary mechanical damage as a result of abuse/vandalism...

If you did not sign a work release for the repair you do not have to pay for it. Any repair over $50 has to have written approval or approval over the phone. As far as getting it repaired, ask around someone may know someone that would repair it on the side or they do it in their spare time. Hope this helps.