I'm after an addon that will basically:
1) Maintain a list of items that I want to check prices on the AH.
2) Maintain a price threshold that I'm willing to spend.
3) When I'm at the AH, search for those items and when they reach (or drop below) the threshold price, alert me to it.
The search could be triggered on opening the AH window or manually triggered at the item maintenance section (I envisage a separate tab).
It could be integrated into Auctioneer or Auctionator. The former would perhaps allow statistical tracing of prices so the threshold could be a percentage rather than a gold amount threshold.
Theoretically, it could be used for selling too to see if prices rise above a threshold, but I don't see that as being as useful as people tend to wait for prices to drop when buying (or at least I do). But it could be used to take advantage of the rich and impatient. ;)
This has come out of the fact that I have taken a massive break from the game and don't have the cash to splash that a lot of long term players have to spend on recipes. I need to be frugal and really keep an eye open for bargains.