There is no way (that I know of) to create a "wish list" inside the AppStore.
All I can suggest is to (1) mail yourself the link to the app and keep a record of it that way. Or, (2) get one of the apps that can can keep a wish list for you (I use the app AppsFire, though there are may others). If you have a wish list in one of these type of apps, you can set it so you can be notified when/if the price drops. Or, just use it to keep track of the apps you want.
That's all I can think of. Maybe others will tell of their way(s) to keep a wish list. Hope some of this helps.
Marilyn