A few years ago, I bought a $20 pair of fit-over type at wal-mart. I expected to lose them kayaking or fishing. for the price they work well on the water, and worked great on the bike too.
If you don't want to go that route, the best prices on glasses is Costco. They had a buy one/get one half off deal, I got one pair of regular glass, and one pair of transitions, for $300. If there is one near you, give it a look.