I'll be honest: I hate gimmicks. Once this App is up and running, I won't give you half off advertising credits unless I really really like you; even then, don't count on it.
Until this App is ready (and it's 90% of the way there already), I'll give you a deal on advertising credits. How will those credits work? Well, that's pretty much the "magic sauce" that I'm not giving away unless you're an investor, which is certainly doable.