Wow that's an expensive appetite! I've never had a belt last shorter than the 60000 mile recommended replacement interval.
Are you getting the same person to install the belt everytime. Are they over- or under-tightening it? Has something caused misalignment of the pulleys? Look down and see if the pulleys line up.
Have you tried a different belt source? Perhaps your supplier got a bad batch.
Are the belts shearing or coming off?
Tyson
95 LS (for sale)