The calipers are *not* locked, it is all but impossible. The *slide pins* are locked. There is a difference, that's just how mechanics distill it for public consumption. It's a market driven price - priced right around the sweet spot of shops making easy money with repeatable results, happy customers, and consumers not wanting to deal with options.
That price is about right - Very roughly it's $50 caliper, $50 rotor, $50 pads = $150 + $150 labor = $300 per side. You want to make sure the shop is using sil glyde or some other high quality cailper grease. the traditional old style grease causes the pin bushings (which subarus and other cars never used to have) to swell and is low grade stuff anyway, but there are still people using it.
You could get this repaired this for about $300-$400 by replacing the slides and pads. This is what I would do and so I'd have you're entire car fixed for $50 in parts (free labor). Here's what you need to repair the "stuck" caliper, buy these and find a mechanic who will install them:
$16 for two of these: https://smile.amazon.com/Carlson-Quality-Brake-Parts-14149/dp/B000C00XI2/ref=sr_1_7?s=automotive&ie=UTF8&qid=1534858754&vehicle=2002-13-66-162--1-6-6-204-108-1-1-2601--6-0&sr=1-7&ymm=2002%3Asubaru%3Aoutback&keywords=caliper+pin+kit
$7 for one of these (this set is for both rear sides): https://smile.amazon.com/Carlson-Quality-Brake-Parts-16083/dp/B000ZN3GNK/ref=sr_1_4?s=automotive&ie=UTF8&qid=1534858754&vehicle=2002-13-66-162--1-6-6-204-108-1-1-2601--6-0&sr=1-4&ymm=2002%3Asubaru%3Aoutback&keywords=caliper+pin+kit&dpID=41khsjLQ5%2BL&preST=_SX300_QL70_&dpSrc=srch
Any average shop in the northeast has to have a torch to function so they have the tooling to remove even a terribly stuck slide pin, you just need to find one that will do it.
If you want - you can even buy new caliper brackets - they're like $20 each, here's one for $25, then there's absolutely no question since no torch or time is needed to free the stuck slides:
https://smile.amazon.com/Cardone-14-1605-Remanufactured-Caliper-Bracket/dp/B003PINW7C/ref=sr_1_2?s=automotive&ie=UTF8&qid=1534859313&vehicle=2002-13-66-162--1-6-6-204-108-1-1-2601--6-0&sr=1-2&ymm=2002%3Asubaru%3Aoutback&keywords=caliper+brackets&dpID=4179Sj5u1ZL&preST=_SX300_QL70_&dpSrc=srch
You spend $25-$100 in getting the right parts and a shop charges $300 labor - and they're making the same amount of labor doing this rather than replacing the calipers.