Our bill has two parts.  Capital Reserve part (which is $750 annual towards the eventual replacement of the pumps & tank in future).  Amount set by board of directors many years ago when we realized we had no capital reserve but tank and pumps had clean bill of health and expected life of 15+ more years.  Second part is based on last year's operating costs + 5% and if we have good years (meaning revenue far exceeded expenses) we go 0% or even -5% to keep the operating costs about even with revenue income (currently $1700 annual).  So we adjust every February based on prior year results.   We have no meters so cannot bill by water use of individuals.