I would suggest contacting the power co. and seeing what it would cost to have them set some poles. I have a hard time believing they'd charge anywhere close to $30000 for that.
Next up, I'd try transformers at each end like MRB mentioned. 480 volt is common, I've never installed a 600 volt, but they are an option. Since they're not as common as 480 volt, the price may reflect it. The higher you can get the voltage, the smaller the wire you'll need to run.
Just running some quick calcs:
480 volt transformers, 5% VD, would get you down to a 2/0 AL I believe.
600 volt transformers, 5% VD, would get you down to a #1 AL or so.
The bad part about transformers is they always draw current while running even with no load on them, plus it's one more potential failure point in your electrical system. Although if they're sized correctly, some of them can last decades. The good news is you can adjust the transformer taps to make up for voltage drop as well. That's why I calculated at 5% VD rather than the usual 3%.
Honestly, I'm very surprised your electrician sent you a quote for $30K without giving you some other options. How many people does he think will bite when given a quote like that?
EDIT: I forgot to mention, you'll only need 3 wires between the transformers. The two "hots" and a ground. You'll derive your neutral from the transformer near your garage panel. Depending on whether or not your electrician figured a 3 or 4 wire feeder from the meterbase/(disconnect?) this could save you the cost of one wire.
