Small square balers always seem to demand a hefty price for ones in decent condition. And the problem is, until you really put it through its paces, you may not know how 'decent' the condition is. Don't assume price reflects condition: Lots of sellers will see balers the same model as theirs listed for high prices and assume theirs is worth the same amount, regardless of condition. Not at all true. Something like a NH 273 in top notch condition could definitely be worth $2500+. A worn one stored outside is probably worth little more than $200.
A 24T is a good (okay-ish) baler. I'd be surprised if you couldn't find a 336 in the same price bracket however, and a 336 is a slightly stouter baler. Nothing wrong with a 24T, but they're getting up there in age, and a little slow: I don't know that I'd want to strike into custom work with one. But I'd still take a 24T in excellent shape over a worn-out 336.
Although there are lots of balers in the $300 to $800 range, if you want a field-ready plug-and-play baler, I'd expect to pay at least $1500 if it's in okay-ish shape and needs only a little work/maintenance. Probably a bit more $$ for a slightly newer model or one in good shape. Possibly a lot more $$ if it's in top-notch shape. Be sure to go over it really well when checking it out. Check all the knotter mechanisms/linkages for excessive wear/slop. Check the plunger crank arm bushings for play. Look closely at the needles for any signs of damage/repair. Look at the shear bolt area to see if it's been modified, or if there are signs of it breaking a lot of shear bolts. Look at the face of the plunger where it would hit the plunger stop, to see if there's a witness of it coming out of time and hitting the stop a lot. Make sure roller chains are lubed with the rollers still turning and the chain sprocket teeth aren't worn to a small point. If it's a Deere (or similar) style with the auger infeed, check the auger and feel how sharp the flighting is. If it's worn sharp, it's seen a lot of use. If a Deere, check the axles for being bent/repaired and the wagon hitch hole for excessive wear: Deere's were notorious for wrecking axles when they pulled a lot of heavy wagons. Not a big deal to replace, but at least a couple hundred dollars for a new axle (don't buy an axle from Deere - they're pricey and have a sharp-edge stress concentration where it's turned down for the outer bearing, which is most of their problem).
If you don't want to spend a lot of time repairing it and don't want any risk, a square baler is not something you want to cheap out on. If I were doing custom work and/or didn't have a round baler as a backup, or if I didn't like doing my own repair work, I probably wouldn't hesitate to spend at least $4 to $5k on one, and folks who aren't as cheap as me would probably recommend spending even more.
Don't get hung up too much on make: Condition is more important than make/model. Both Deere and New Holland, of course, made an excellent baler. You're probably safest to stick to Deere/NH, but even Massey's were pretty good if tuned up correctly. I have a 90's era Deutz square baler that uses the same Rasspe knotter as Deere's/NH, and it's a phenomenal, heavy-built unit. Got it for $2000 a few years ago, which was a bargain for how good it is.
If you're mechanically-inclined and don't mind a project, the $300 to $500 might be worth it. I have an older 70's era Allis 442 I use at my father's: The Allis's were very good balers and also used the same Rasspe knotters as NH. I got it for $300 about 8 years ago. But it required a plunger rebuild (new rollers with proper alignment) and I had to cut out and weld in new tracks where the old ones were rusted/bowed from sitting in the weather and having water freeze between them. Plus some reel bearings, new roller chain, crank arm bushings, and knotter arm bushings. It probably cost another $500 in parts, but it doesn't miss a beat now: I don't think it missed a single tie last year. I'd probably trust it more than most other balers, simply because I've been over it well and know exactly what I've got. But it took a fair bit of work to get in that condition. The only balers I'd stay away from are the IH or New Idea (rebadged IH). They were actually built very solidly and robust, but they used a wacky, 'All-Twine' brand knotter, not the Rasspe style favoured by most others. If adjusted and run right, they'd work well, but you really had to hold your tongue just right and be persnickety about maintaining knotter adjustment and knife sharpness constantly.