Yes, it makes perfect sense to me and I will not be surprised if you find an inverted U-shaped relationship in your test.  At the low levels, it will not be so effective and the impact will increase until a certain level.  After that, some saturation will kick-in and sales will not be effected.  There are lots of literature on decay or saturation of marketing mix and you can easily find them form a google search.

Thank you for the feeback, Muammer Ozer.