As long as you're not getting it for self-defense, I'd say $300 is a decent price.
I paid just under $400 for mine if I remember right.
I had quite a few problems with mine at first, but a lot of it ended up being that the bolt wasn't closing all the way. I replaced the recoil springs, and it's actually fairly reliable most the time now.
Accuracy is absolute crap. From 10 feet away I can't even hit a pop can with it (and neither could anyone else who I let shoot it). A lot of them came from the factory with misaligned sights, which seems to be the problem with mine.
The finish on them is also terrible. Mine starts rusting even when it's sitting in the case in a house, so I wipe it down at least once a month whether I shoot it or not.
They're uncomfortable and awkward to shoot. You're not supposed to hold onto the mag while shooting it, and they're really heavy for shooting by just holding onto the grip like a regular pistol. I once tried bump firing it by hanging onto the barrel shroud (it's a wonder I didn't accidentally shoot myself in the hand or something), and ended up cutting the hell out of my hand on the sling catch thingie. I failed at getting it to "bump" too, which was even more lame.
They're a pain to dissasemble (for me at least). The rear cap unscrews (which gets marked up everytime you do it) to strip it, and when you put it back together, it usually binds up so the gun won't charge and you have to try again.
There's no good aftermarked mags for them. Western, National, etc. are all garbage. If the extra mags with the one you're looking at buying are factory mags, those are actually pretty expensive on gunbroker, and the only mags that (usually) work reliably.
Only shoot standard pressure 115gr fmj in it. Heavier bullets and high pressure loads supposedly crack the receivers.
These guns get really dirty after just a few rounds for some reason also. Poor design?
Spare parts are expensive. SKSparts and stony creek armory are some of the few places that typically carry parts for them. Stoney creek armory also sells drums for them, but they're like $200, and I don't know if anyone has actually bought one. They also run a message board for tec-9s that has the best info about them that I've found so far.
Despite all the bad things that people say about these guns (most of it well deserved unfortunately), they're still one of the coolest looking and most infamous guns ever made, so hopefully you get it.
