Shaun,
I have a fair idea of the price... I tossed up several options when developing my discount voucher site www.EveryOunceCounts.com.au -
We decided to go with a web app, mainly because I could cover multiple handsets for the price of the standalone iPhone app. Also not being all that familiar with the iphone sdk myself, the web app allows me to update the single website for future versions.
I have to say, the 'learners app' was something we tossed around at a bbq a while back, great idea! - I wish you lots of luck and success.
After spending a few grand on some Facebook tests, I would agree its better to do a lot of small campaigns with different ads. Competitions also improve click-through rates and can be quite effective.