Adventures typically has ball locks for $40. I picked up one that needed a lid and the rubber handle glued down for $10 there recently. I got a brand new lid at the same time for $15 $25 and a little gorilla glue and I ended up with an other keg.
I got one from adventures a couple months ago. They are the cheapest I could find. And all in all I was happy with the transaction. But hollymoly was it ever crudid up with syrup. I had to unscrew the post from the keg take the poppets out and soak everything for a week in cleaner.
I bought two ball lock kegs a couple months ago for $30 each BUT... they nailed me $30 for shipping. So, that averaged out to $45 apiece which seems to be the going rate. I will say these were in excellent condition and to my surprise arrived pressurized (guess to prove they didn't leak?).