CPM cost of an audience is based on the number of people you’re telling the platform you want to reach. For broad audiences it’ll be cheaper for hyper targeted or smaller audiences it’ll be more expensive.
Does the CPM really matter? if you are driving leads, then the ultimate cost metric is Cost Per Lead, or Cost Per Result in FB language. As long as you have a clean conversion set up for a lead opt-in, then you should gear the campaign toward that and focus on the CPL/CPR cost metric.
With that said, a $20 CPM seems pretty high. But if you’re CPL is producing a favorable ROI, then who cares what the CPM is?