This is a labor intensive job. You can call around to find the best price. Here is the procedure for repair and you can make your determination http://www.ehow.com/how_6853026_replace-timing-belt-mazda-millennia.html Labor should be about 4 or 5 hours so the price does seem a bit much. Did you go to a dealer or a shop? Dealers always way overcharge for service and parts.
Diagnostic would tell about valve damage possibly. The belt would show excessive wear or be broken thru a visual inspection or the diagnostic could show a timing fault. If you have valve damage, this is a fair price for the complete repair.