When you say "dirt cheap", you're actually dealing with two very different categories.

There are the cheaply made variety, with poor optical performance and comparatively flimsy, less than precise construction.

Then there are those that are not cheaply made, but are older, manual focusing lenses that can be found for little money if you know where and how to look. For example, a 1980's 400mmm Nikkor lens. Optically, a decent performer, and certainly not cheap when it originally came out. If used correctly, it will yield excellent results.

Just a thought. I think that this will be a fun adventure. As I suggested in another post, check the "junk bins" in local camera stores, if there are any around you. This may also require the use/purchase of an older camera like my Pentax Spotmatic from the early 70s. For current digital use, there are those that can make/provide lens mount convertors. Good luck & have fun. Let us know what you end up with and show some photos! I love my expensive stuff too, but photography should also be fun.

I have both a 400mm and a 500mm preset lens. Both work well enough if I have time and a tripod. Neither can really be handheld even mounted on a Pentax K100d (a really old 6mp camera) with in-camera stabilization. I set the camera to "A", aperture priority, set ISO in full sun to 200-400 and let the camera set shutter speed. On cloudy days I set the ISO to 800. I shoot in RAW and pp to get the most.

Both lenses produce sharp images if I get the focus right. I have better luck near infinity where I can just back the focus off a little. The 400mm produces better images than the 500mm. I paid $.99 (99 cents) for the 500mm, and $9.00 for the 400mm on ebay, plus postage.

I have posted images from both on this site.