$300 after taxes etc from O Reilly is insane. Are there any other options pref via Amazon? I'm in a super remote area with very few options but Amazon does deliver.
Yes it is. I've replaced the one in my Gen 2 Prius a couple times, and that's what it cost. I recently had to replace the one in my Gen 4, which is under the hood but it's still in that price range. The really insane part is that after looking at the 4 different auto parts dealers here that sell a battery for my Gen 4, I decided to see what the Toyota dealer had. The OEM battery cost about $50 less, had a better warranty, larger capacity and more cold-cranking amps than any of the aftermarket batteries. Oh, and they had one in stock. I think if I ever lose the battery in my Gen 2, or Gen 3, I'd go for the OEM. At least give it a try.
Normally I tell people to avoid Amazon for autoparts, especially elecrical ones because they sell so many dangerous counterfeit parts. An AGM battery might not be too bad though. From a search just now, it doesn't look like Amazon has the S34B20R battery at all. They do have a few in the $300-400 range that are close and could be adapted to work, but that's too much money for too much hassle. Advance auto parts is selling the right battery for $255 and they offer free home delivery. You are likely to pay a bit more because of the recycling deposit. You'd get that refunded when you take the old one back to the store.