Tile can sound hollow for a variety of reasons. Sometimes there isn't a problem and that sound is due to the configuration of the tile assembly below it. If some tiles sound hollow and some don't, then that is an indication that some portion of that tile assembly is not bonded well or there are excessive voids under the tile causing that sound. The only way to determine which condition it is, is to intrusively remove some tiles. When there is a problem it is never due to a single deficiency but rather due to a number of compounding deficiencies. There could be excessive moisture, there may be a lack of movement joints in the installation, the tile may not have been installed correctly, the tile may not be adequately bonded, etc.
Whether $2,500 is enough to fix it depends on the configuration of the tile assembly, how many tiles need replacing, and the extent or nature of the problem. Is if isolated problems or global? Will it continue to grow worst? Are they planning on fixing the problem or only the symptom?
We do have a lot of experience at investigation these types of problems. We have an inspector in Miami, FL and near most major cities in North America. It may or may not be practical for you to utilize our services depending on the circumstances of your situation.
I would need to know more about your situation to determine the scope and cost of the investigation.