Note: my free account (where I was testing this) ended a long time ago, so I'm unable to add a field to this. I would add "How much will you charge for each appointment?" I would do this with another slider going from $0 to $300 and defaulting at $100. In this way, even if they move the monthly cost of BWS to $500, they're seeing $6000/month profit based on 40 clients per month. (2 clients per work day.)

Obviously this is in US dollars; you could have a unique calculator for each region.

In short, this is just to show you how it works. You could recreate this and improve on it at calculoid.com ​or some similar service.