Perhaps a slider or some other interactive tool would be better for estimating monthly cost. Anyone who'd use your service can look up their existing active us

Perhaps a slider or some other interactive tool would be better for estimating monthly cost. Anyone who'd use your service can look up their existing active user counts and plan accordingly -- much easier than estimating data points. However, you're going to lose some people if you require mental gymnastics to get an estimate based on arbitrary tiers and pro-rated pricing.