at $20 bucks, I'd buy it and get it hydro'd.
I'm not sure what your LDS charges, but most dive shops charge $20 to $25 for a hydro and $5 to $15 for a visual. On average you can get it back for hydro($25), vis($10), and air fill ($5) for about $40 bucks total. You might be able to take it directly to your local hydro facility and get it done cheaper. I can get a hydro, vis+, and air fill for $19 bucks at my local hydro facility.
If it was really dirty they might have to clean it. Figure maybe $15 to $20.
Does it have air in it now? If so that is a good thing as it prevents moisture from getting inside the tank. It also means that your valve is not leaking. (I've bought used tanks before only to find out they had leaky valves and had to replace them (about $45 bucks)). I just mention this because used tanks aren't always a bargain if you've got to spend a lot on them after you get them. I once bought some used tanks, got them hdyro'd, visual'd, only to find out the valves leaked also. By the time I spend all that extra $$$, I could have bought a new tank for about the same price.