From March:

The decision to deploy 1,800 AITs represents investment of about $300 Mln over period 2010 – 2014 (procurement only, not including training, installation, and maintenance costs). Staffing these machines will cost another $220 Mln. Training, installation and maintenance cost will likely add another $150 Miln into equation, bringing total the bill for deployment of new screening machines to about $700 Million over six year period.

In the 2011 appropriations bill they state they renegotiated the warranty on the units from one year to two years, that may be why there is not a service contract on them, they are under warranty

SCREENING TECHNOLOGY MAINTENANCE AND UTILITIES

The Committee recommends $323,325,000 for screening technology maintenance and utilities, $8,800,000 below the amount requested in the budget and $6,700,000 above the fiscal year 2010 level. The recommendation does not include $8,800,000, as requested, for maintenance of new AIT units. This amount is no longer necessary in fiscal year 2011 due to TSA's re-negotiation of warranty contracts from 1 year to 2 years

I would expect then, as each machines comes off it's warranty the costs will explode to keep them working.