High Efficiency Toilets

[JR T.]

I have a number of multifamily buildings that are 100+ years old and in the market where they're located I pay for the water. One three unit building is using $1,200 per quarter in water. I believe part of the issue is the old toilets which use more water. Even when we've replaced the flappers if it doesn't drop down exactly right the toilet will run. 

What are my options for high efficiency toilets? Will the toilets that flush under a gallon each time be compatible with the varying 100 year old plumbing that is in these structures? I've seen toilets that do not have flappers in the $250 range - does whatever they're using wear out and allow the toilet to run? Any other ideas?

I'm willing to spend some money to upgrade these things because I will make it back on the water over time. Please help me figure out the best product for my needs. THANKS BP!