Chip, I looked into the same thing a few years ago when I first moved into my place (which had a rented tank). The supplier charged MUCH more for fuel than other competitors. At the time they wanted about $1200 to buy out the 500 gallon tank (which was in pretty rough shape). This was the same price as a new tank (they did not take into account that it was a used). I had them pick it up (was not using propane at the time) then last year revisited the idea. Found out that tank prices had gone up a bit in since I first looked into it and were now up to a little over $1500.
I shopped around craigslist and came across a deal on a 500 gallon tank for $500 dollars. I sanded, painted, and replaced all the valves on it for about $150. So for $650 and a little work I placed my own.
No kidding, on the first fill up I saved enough to pay for the new tank outright.
They do not come around all that often but if you go to google and put in "craigslist 500 gallon propane" it will give you all of the listings for tanks (not just what is in your imidiate area, like you'd find if you searched your local craigslist site). This is how I found mine (it was about an hour away from where I live).
Hope this helps,
Brad