Quality UL rain jackets tent to be really expensive, but I've seen wind shell jackets for very reasonable prices. Take for example the Massdrop Veil Wind Shell at 2.7oz. Granted $60, plus shipping, still isn't exactly cheap, but it's much less than the typical $100+ for rain jackets. So, I'm wondering if anyone here has ever tried to rain proof a wind shell and how successful you were? For an even more affordable solution, you could look for a local store brand wind suit top, or a similar top from a second hand shop, tear out the liner and waterproof that instead. I think some/most of these are already coated with a water resistant finish, so maybe you'd have to soak it in something (mineral spirits?) to get a new coating to stick.

I'd love to try my hand at a DIY rain jacket with something like this, but I don't think I'm ever going to get around to it.