Thank you, everybody for all the recommendations!
After going through them, it was looking like if I wanted an OpenWRT device for ≤$50 US, I'd be buying either an Archer C7 or a GL-AR300M. Amazon sells refurbished C7s for $50 and new GL-AR300Ms for $40, both with free shipping. The average price of used C7s (including shipping) on eBay was $35 in the last few days.
| Price | Flash size | RAM | WiFi | Bands Archer C7v2 refurbished | $50 | 16MB | 128MB | AC | 2.4 & 5 Archer C7v2 eBay | $35 | 16MB | 128MB | AC | 2.4 & 5 GL-AR300M | $40 | 128MB + 16MB | 128MB | N | 2.4 GL-AR300M-Lite | $18 | 16MB | 128MB | N | 2.4 GL-AR750 | $45 | 16MB | 128MB | AC | 2.4 & 5
I very much liked the GL-AR300M as it comes with OpenWRT by default. In fact, its page on Amazon is basically a panegyric to OpenWRT, touting its beautiful strengths and universe of features. Its clear this router was designed for people like me. It comes with dual flash areas (128MB NAND, 16MB NOR), but you can get it for half the price if you opt for the "lite" model with only 16MB flash.
However, I don't need a pocket sized router and rewiring the installation site to provide 5VDC power over a MicroUSB cable would be inconvenient. The GL-AR300M's lack of 5 GHz would be a serious problem in the congested urban area where I'm planning to put it.
End result: I'll likely be getting a used Archer C7 v2 for this installation. However, at a different location, it looks like I'll need a much beefier device, so I'll be re-reading this thread as I compare the more expensive routers.
Edit: Added GL-AR750 to table. If the 128MB NAND isn't usable on the AR300M, the slightly more expensive AR750 seems promising: it has a MicroSD slot for extra storage. Other nice features: 5GHz band, WiFi AC, and (optionally) power over Ethernet.