CPM for mobile ads

I'm wondering what influences the CPM for ads in mobile apps. I'm fairly new to the whole ad game so any insight into CPM basics is helpful. For example monthly, annual and seasonal variations.

I have two free iOS apps. Call them A and B. A is a novelty app that's been in the app store for a year. A has about 500 downloads total and currently gets about 5 downloads per week. App A has zero ratings. So IMO it's safe to say app A is pretty much dead.

App B on the other hand is doing relatively well. It's been in the app store for six months and has around 15K downloads. Currently it gets about 100 downloads per day. There are approximately 2000 daily active users who consume about 5000 items in the app per day. Assume the items are photos or tweets or similar. The number of "items" consumed per day has more than doubled in the last 4 months. App B has a five-star rating over about 100 ratings. So IMO App B is doing relatively well compared to app A.

Here's what's puzzling me: app A has an eCPM around $15 while app B has an eCPM around $2. What could be influencing this massive eCPM difference?

CPM rates vary by vertical according to the number of advertisers. The more advertisers there are, the higher the price.

For example, there are dozens of online brokers promoting a trading account/platform, so anything related to stocks or investing will have a high CPM. On the other hand, there are fewer grocers advertising online, so those ads cost (and pay) less.

The trick for you as a publisher will be to create a popular app that's in a vertical with lots of advertisers. If you have an AdSense account already, you can surf there to see which advertisers are paying the most. Or you can create a free AdWords account to see how much keywords for certain verticals cost, that'll give you some idea.