Creating a custom transaction?
The prices you present are pretty steep, but hopefully this is just for clarity;).

If you don't want to run a daemon yourself in a loop, you can always try using Strongcoin's API:

https://strongcoin.com/blog/bitcoin_micropayments_with_strongcoin

It sends notification to your website when a transaction that you are interested in appears (whether it is to one of your addresses or anyone else's). It might not necessarily be exactly what you want (say, if you require 6 confirmations), but it definitely is a good place to start if you just want some notifications.