Thanks for reaching out with your question about your crepe myrtles. Having the root collars buried too deep can definitely lead to the premature decline of your trees. I tried to identify places around DC that might have an airspade or similar pneumatic excavation tool available to rent without too much luck. There are some places in Pennsylvania that have them for rent, but having to travel a long distance to pick up the tool could potentially make the cost nearly the same amount that you've been quoted by the arborist. They also require fairly powerful air compressors, which is an additional cost to consider. 

They seem to be a tool that many arborists have, but not as easily available for individuals to rent because it is a tool that requires specific training, site preparation, and safety precautions. For example, AirSpade stresses the importance of measuring the soil moisture  and doing site preparation to remove turf/protect existing plants. In addition, when using an airspade, safety is paramount. Blowing dry dirt can create hazardous conditions and a risk for airborne stones within a 25 ft radius or more, so it's essential to wear protective gear such as a full face respirator, face shield, and appropriate ear protection to protect yourself from dust inhalation, rocks, and loud noise.

There is some helpful guidance from Penn State Extension on how to address the issue of having a tree that is buried to deep as well as the potential root girdling that can occur as a result. 

If the root collar is not too deep you could try to use a hand spade, but there are potential risks to the tree. Digging out a root collar by hand does make it more likely that you could damage the tree or the roots. Additionally, please note that this kind of work is best to do when it's either fall or early spring when the tree is not leafed out so that the stress to the tree is minimized.

Although the cost is significant, this may be a job that requires an arborist with the proper tools to help ensure the long term health of your trees and to minimize stress and damage. 

Please feel free to reach out if you have any additional questions.