I'd happily share my experiences with promoting. I've had a little go at promoting in both AdMob and AppBrain...
I first tried a very small experiment with 'Happy Chick - Flying Game' on Android, in AdMob. My budget was £10 a day over 3 days and this resulted in the following;
Impressions: 710,546
Clicks: 2,930
Conversions: 33 (Although I'm not sure how they report this as we didn't get that many actual downloads)
Cost/Conv: £0.89
I then thought I'd try the same game on iOS, only this time I ran it for only a day (same budget)
Impressions: 1,629,822
Clicks: 1,150
Conversions: 0 (Although I think this is because AdMob cannot actually track iOS without the necessary SDK. Going by my download stats though, 0 wasn't far off lol)
After this disappointing run, I thought I'd try a service which offers 'Cost Per Install', as opposed to AdMob's 'Cost Per Click'. For me this was abit of a no brainer, why would I want to pay someone just for clicking my ad? I'd much rather pay only when the app is installed. Anyway, AppBrain offers this, so AppBrain was my next experiment...
I thought I'd up my game (a little lol) for AppBrain, and added $100 with a daily budget of $25. I also went with my other app 'Double Jump' (Android). I was messing around with my bid over the first couple days, but eventually settled with around $0.25 per install... This was the result...
A bid of $0.25 is very very low (might have actually been the lowest), and from all the installs I got, the vast majority were from Iraq/Iran... I'm told that to hit markets like UK/US, your bid needs to be around the $2.00 mark... (which would have resulted in only 50 downloads with my $100 budget!!).
I know this is only a very very small sample, so should be taken lightly, but I thought it'd be good to share my experiences anyway, seeing as you asked
In my opinion (and I've read this many times), in order for 'In App Advertising' to be successful, you have to pump thousands into it!
I would however recommend AppBrain over AdMob should you wish to have a little go yourself... Good Luck
Using AppGameKit V2 Tier 1