You could work with the installing contractors to perform the prefunctional checklist/inspections (See EAp1; STEP 8, the first bullet) in the 2009 Reference Guide. However, there is no reference to projects 15,000 sf or less being exempt.
I have also never heard of any project, no matter the size, being exempt. This is a problem for smaller projects, there are many tasks in Cx that are not related to size...you could have 1 hour of performance testing, and yet you have to develop a plan, develop specifications, matain a log, and make the LEED submittal.
I would be surprised if you would need $15k for Cx. We have done similar projects and been below $10k. Note that for this size of project, someone on the design team can do the Cx if they have the required qualifications as required by USGBC (ie at least 2 projects as Cx).
I would recommend that you see if the mechanical designer has the required experiance, then work on a plan that would allow them to direct the process. The testing should be fairly straight forward based on your description.