From the Wayback Machine, we can track the increase in price of both new and used kegs. NorthernBrewer.com is a good resource for this because they've been around so long. (And they deserve it. You guys rock!)
April 2001 to May 2006: New $95.00, Used: $30.00
May 2009: New $120, Used $35
May 2010 - Unchanged at $130/$35
March 2012 (present) - $130/$45
So, yes, you're right that there's been a recent jump in prices. One would also posit that the jump in new keg prices between 2006 and 2009 is due to fewer new kegs getting manufactured for the soda market; soda is gigantic compared to homebrew, as Warren Buffett can tell you.
It's likely that for years there were untapped reserves of used Corny kegs that could've been sold in bulk by soda distributors to finance conversion to the newer bagged systems for syrup. The less popular pin lock kegs have been keeping pace with the ball lock kegs. My recent research shows that the price of a pin lock keg plus a conversion kit was about five bucks short of the commercial price of a ball lock keg.
This is a pretty efficient market, populated with people who are resourceful and thrifty enough to homebrew. We can be relatively sure that we're free of distortions.
As the supplies of refurb kegs dwindle, price goes up. As prices rise, there are a few inflection points. The first comes when the price of a refurbished keg rises to the point where a brewer sitting on unused kegs has significant incentive to take on the transaction cost of selling them.
The price of gold has spiked recently. You might have a pair of earrings with no sentimental value that you wouldn't have bothered to sell when their half-ounce of gold would've netted you a hundred bucks, but now that they're worth four times that, you might get them appraised.
A lesser effect might be seen first with refurb kegs and then with "used new" kegs.
My gut says that the first inflection point has to be around double the historical retail cost of $30. So, we'll see some downward pressure on used keg prices as people start remarking more and more about the cost of used kegs. I have to think this tops out at about $60. Here's why:
Kegs are a durable commodity good (one's basically as good as another, and a used one works roughly as well as a new one). I don't have ready numbers on expected premium for new vs. used durable goods, so I'll have to approximate here.
Right now, we have a soda-surplus refurb keg around $50 and a brand new keg at $130. There's little reason for someone to resell their new keg, because they have to compete with all the other refurb kegs which also have the taint of prior ownership.
As refurb kegs become more scarce, a different market opens up. Let's say you bought a new keg a year ago at retail for $130. If you asked $100 for it, which is a reasonable discount, the number of people clamoring to take your offer would depend heavily on the price of refurb kegs. Few would take it with refurb kegs at $50.
Why pay double for something that's basically the same?
Once refurb kegs hit $80, though, the thinking is different.
What's an extra 20% for a better-quality keg with only one previous owner? It hasn't spent its life on trucks being bashed around by guys with "Jerry" embroidered on their jackets. It's been sitting in a guy's basement dispensing Simcoe pale ale.
Leaving aside the question of whether new manufacturers will emerge and push down the price of brand new kegs, the likely ceiling for refurb kegs is around $70. Plenty of pressure from brewer resales on the way there, then competition from resale of never-used-for-soda kegs manufactured for brewers.