As you've found out, custom framing is extremely expensive.
BUT - it's not hard to do yourself, and you can get archival quality for about 1/3 the price (and even cheaper if you don't care about UV-blocking glass/plexi)
I buy stock-sized pre-cut mats and backer online (I use
www.redimat.com). Stock sizes are much cheaper than custom, though they offer custom-cut too. Instead of glass I typically use 1/8" plexi, which is easy to cut at home with either a bandsaw or even a sharp utility knife and straight-edge - if you need to cut at all. UV-resistant costs ~2.5x more, and UV+non-glare is about 4x more.
For frames - there's a lot of cheap wood frames out there (again, reditmat but also from places like target). Keeping it stock keeps cost low.
A few months ago I framed up some large-ish vintage prints (20x24"?) with UV-plexi and archival mat and backer and a maple frame I made myself. I think my out of pocket cost to do all three was around $150. CostCo wanted something like 3x$120=$360 + tax for archival+UV. Frame shop was even more.
Had I gone with non-UV plexi it would have been closer to $60 out of pocket for 3 prints.
Building your own frames (even fairly complex ones) isn't hard if you own a miter saw/tablesaw + router.
https://www.youtube.com/watch?v=RmyytuR-iVEyou can use much nicer wood than the stock pine or MDF budget frames sold in Target or Walmart.