Actually contract towers are the outsourced operations, using private companies like Midwest ATC and Serco. These are not FAA towers. There are 254 contract towers in the US.
The government audit of these towers said that on a like for like basis FAA manned towers are four times more expensive and in effect are over manned.
The audit can be seen here:
DOT - Contract Towers Continue To Provide Cost-Effective and Safe Air Traffic Services, but Improved Oversight of the Program Is Needed | Office of Inspector General if you are involved in ATC, have a look, it makes astonishing reading.
Meanwhile the government politicisation of the FAA budget means that even contract towers are affected.
There is a big policy push to get this changed. See
Reason Foundation
FF