Examining the rationale for this slightly …
I am guessing the reason Admob prohibits “encouraged clicks” (incentivized clicks) etc. is that it distorts the click value of the ad - in it’s worst form it can be excessive clicking just to deprive Admob/advertisers of any value while developer collects the revenue.
If one understands this, one should be clear that this is not prohibiting users from showing incentivized ads or asking users to click on ads - just don’t do it with Admob (as far as Admob is concerned).
However, the question you ask is a persistent thread of concern for developers - how do we have users reward us - i.e. micro-payment (without actually doing in-app payment etc. - may not be supported in your are or you may just want very small amount per user which is not supported by current Google payment models ?).
For this you can use the virtual currency solutions like GetJar/Tapjoy etc.
These allow you to blatantly ask users to click on the ads/download the apps from appwall etc. in return for in-game currency/coins/gold etc.
If that is the feature you are looking for, then Tapjoy is the easiest to incorporate (GetJar has “potential” but serious issues for a naive developer) - users are also very familiar with Tapjoy and it’s look.
However, just like the conversion rate for in-app purchasing, the situation is similar for GetJar/Tapjoy virtual currency alternatives - i.e. it is very hard to get users to actually “do some work” to get something. In fact it may even be easier to charge them cash (if you count the contribution of “whales” i.e. with cash you can ask for $5 - and the few “whales” can tilt the balance so that in-app purchases could get you reasonable revenue). That same situation does not exist for GetJar/Tapjoy etc. where there is no concentrated (pre-available in the pocket of the user) cash - that they can use without doing work - instead GetJar/Tapjoy require manual labor on the part of the user to “earn” the coins. This feature of virtual currency ruins the ability to charge the “whales”.
So the conversion from GetJar/Tapjoy will be perhaps not that huge.
HOWEVER, you can perhaps tilt the numbers in your favor IF you have a game or some seriously great features - then it is possible that one could get good conversion.
AGAIN, however with GetJar/Tapjoy - the full potential will still suffer (compared to cash and the whole “whale” user thing) - that out of 10K users of your app - perhaps 1K are serious fans. When you ask for $1 - these 1K maybe willing to pay $5 even. With in-app purchasing you could charge $5 (I think I have seen games offering 250K coins etc. for $99 - though these are casino type games).
So this will always remain a weakness for the GetJar/Tapjoy model - i.e. you CANNOT overcharge for your app - but have to offer features at reasonable prices i.e. “download 5 apps” type of user labor - and you cannot ask users to “download 500 apps” to earn enough coins so it pays you $5 etc.