Yes, we agree and are looking into this feature.
We're thinking of something that keeps it simple, however, and simply lets you flag whether you have a coupon for a particular ingredient. Currently, we're not hearing from too many users that they want to go through the trouble of entering all the data (% or $ off, expiration date, 2-for-1, etc.). In general, the more detailed data-entry is required, the less we notice the vast majority of home cooks embracing a given feature.