If you plan to use advertising to promote your iPhone app, don’t expect to break the top 100 without paying a pretty penny.

To do so, it will cost $1,875 a day, according to Adwhirl, which aggregates a handful of ad networks to help publishers monetize their apps on the iPhone. (If you remember, Adwhirl recently got attention for sparing with AdMob about its policies on aggregators).

Since paying that big sum is not practical, Adwhirl has another suggestion: develop a free iPhone app that will generate a large user base, and then leverage those users it to drive traffic to your paid app. The idea is not super original. Game companies commonly offer free versions to get people hooked in hopes of converting them over to paying customers. However, Adwhirl does a good job of explaining why. More on that after the jump….