TP-Link has put out some cheap but good OpenWrt-compatible routers:
At the very low end, the venerable WR841N (2.4 GHz only 802.11n up to 300 Mbps) which can be had for under 20€.
Then the WDR3600 (or: 4300, 4900) (dual-band 802.11n up to 600 or 900 Mbps, much beefier CPUs) which are end-of-line but might be found used for about 40€.
And from their current line-up the Archer C5v1 / C7v2 (note the hardware revisions, which *must* match; note also that C5v1 and C7v2 are hardware-identical) (dual-band 802.11ac up to 1750 Mbps) which retail for 60€ to 100€.
There are others, but I can’t speak for them. The TP-Links are pretty well-known for their value for money, for example, the German community wifi “Freifunk” has several tens of thousands (!), primarily WR841Ns, all running OpenWrt.