Hi Daniel,
The idea with the eCPM settings is that you enter the eCPM performance of each network, and the mediation ordering is the highest eCPM network goes first. So in your situation, whatever network you set at $5.00 got the first look.
In your situation, what you really want is to check the "Optimize AdMob Network" box in your mediation settings, and set your house ad's value to something reasonable. Then at ad request time, AdMob will run an auction and automatically place itself in the correct spot in the mediation order based on what value it can get in that request. If AdMob can beat your house ad's value, it'll serve. If not, the house ad will serve first.
Thanks,
Eric