This is one of the many spurious claims made by fluoridation exponents. One would think that the parties making such a claim would be obliged to back it up with adequate proof. The fact is, they can't. Numerous reviews have attempted to find evidence of financial savings and all have turned up empty.
Several examples follow.
Douglas et al. Impact of water fluoridation on dental practice and dental manpower. This is the only direct comparison of dental practice and costs in fluoridated and unfluoridated communities reported in the dental literature. It showed no significant differences in the cost and nature of dental care relative to fluoridation. In fact, dentist's income in fluoridated communities was slightly higher.