What do you consider "cheap"? I'd think any baler that is field-ready and makes a good bale and isn't about to fall apart would be worth $4000 or so.
You could buy something "cheap" that someone is tired of fooling with . . . something that won't tie or misses a lot of knots . . . something that makes banana bales, etc. But it's hard to know what you're going to spend to get it running . . . IF you can get it running.
There are quite a few New Holland and JD balers out there. I'd stick with one of those. Tell the seller that you want to see it run. Take 5 bales of hay with you or ask if he has some available and run them through it to see it in operation. I would expect to pay about $4000 for a JD 336 baler that is truly field-ready. I would advise to NOT buy anything older than a NH 273 or a JD 336. Those balers are already 50 years old.
You'll find balers that don't run well at all priced about the same as ones that do. It's hard to know unless you actually see them run. A "cheap" baler may not be the one that costs the least.
We ran an Oliver 62T for many years. Pulled it out of the previous owner's woods for free. Took a lot of work to get it going, but we managed it. But we had nowhere to store it inside for the winter, and eventually, something like 30 years later, rust took its toll. We cobbled it back together again and again enough to run for a good 10 years longer than we should have. But in 2020 we picked up a piece of metal that should have broken a shear bolt, but collapsed the chamber instead, and that was that.
With hay still on the ground, we made an emergency hunt for another baler, finding a 336 just three miles away. It had baled a couple of years before, but was worn and had had some issues. And it had a thrower on the back but we didn't have the wagons for it. (We still pulled bales out of a chute to load them.)
He wanted $3200, but we were desperate. And the owner was throwing in a second "parts" baler with enough good parts that it supposedly could have been made functional again, too. And he offered to tow them both from his place to ours for free.
We took it. Used our Trump Covid payments to pay for most of it, so technically they went into supporting the local economy as they were supposed to. Pulled the thrower off, and modified the chute from the Oliver to work with the 336. Needed some work here and there, but we baled some hay while parked after one day, finished baling the hay that was down the next. Even though it had been used with sisal twine, the knotter worked almost perfectly with our plastic twine from the start.
Still using it, towing it behind a JD 4600 tractor. We've had people nod knowingly and tell us that tractor is too small for that baler, because it's a "compact utility" tractor, but the manual says 40HP is enough, and we say a 40HP tractor is a 40HP tractor, "compact utility" or otherwise. Without the thrower, and with smaller wagons, and not baling at capacity because the old guy on the wagon couldn't keep up if we did, you don't need as much tractor as one might think.