I know you have seen me post around here before, so this is not spam. I have no affiliation with the App I am about to detail. I am also not promoting any kind of hacking or cracking of the iPad or the apps.
AppShopper - Free
AppShopper allows you to create an account that you can list all your current apps as well as create a wish list of apps you want.
Once you create your lists, you can set it to alert you when things change with the app. For instance, I was watching NFS-Shift for it to drop in price. I had it on my wish list. This last round of price reductions by EAGames prompted AppShopper to shoot me a message with the price decrease.
I also set up a view for "Popular" that lists all the latest changes for the apps that are being watched by AppShopper (this is different from the Wish List).
On the left column, select "Popular"
Then, working our way down, select "Free" in the Price section
Then select "Price Changes" in the Type Section
Finally select whatever category you wish to check.
Doing this, I have not paid full price for probably 90% of the apps I have gotten. Sure, it can take some time to set up all your apps that your have installed, but it will watch them for changes and updates and will alert you when that happens.
This is just my own personal endorsement for this app. It is great and has saved me hundreds of dollars so far.