How to discuss chargeable effort in SCRUM?

How about a flat fee per Sprint and that they can decide it is done at any point? Promise them that they will have an ability to see the progress being made at the end of each Sprint cycle and the opportunity to provide feedback that can adjust the work being done to deliver them what they want when it is done rather than what they wanted months ago when they first talked to you. Let them decide when the work you are doing is enough.  Rather than doing a fixed length contract, do a value delivered contract.  It could take one sprint, it could take 35.  You don't know and you don't really care.  The customer gets to direct the development along the way to get what they want and need.  It would be more akin to having an inhouse development staff than having a contracted one. 

That is the benefit of iterative development that Scrum is based upon. 

If you start with a Scrum Team - one Scrum Master, one Product Owner, and Developers - you should be able to figure out how much this team costs per unit of time. Since a Scrum Team is usually stable over time, the cost should be fixed. In my experience, organizations that charge clients for time usually have costs per type of role and a multiplier based on experience, so if you know the skills that are needed to design and develop the product, you can at least estimate the costs for your Scrum Team.

From here, you can figure out an appropriate length of effort. In my experience, your clients probably won't want to pay by the Sprint. Instead, they may want to pay in months or even years. If your know the cost of your Scrum Team per unit of time, you can figure out how much to charge your client over the duration of the effort.

Since Agile methods are about responding to change, your agreements may need to address things like what would happen if the work ends up being less than anticipated at the beginning or how to go about extending the relationship. Since you're reviewing progress and changing direction at least as frequently as once a Sprint, scope changes are inherently built into frameworks like Scrum. Time is the variable.

The most basic unit of accounting is the Sprint, it ought to be possible to work out how much it costs to run X team members for Y Sprints. The more Sprints a client purchases, the more innovation runway they have to control their complex delivery risks and to maximize product value.

Contractual terms may be introduced which allow more Sprints to be purchased at the given rate, should they be needed. Conversely a refund may be offered on a percentage of the remaining contract, should too many Sprints have been bought and value and risk were optimized earlier than anticipated.

Dear Rafi. I suggest you to google, and the carefully read Evidence Based Management Guide which is available at scrum.org. There are many answers, including detailed guide how you can build a matrix of calculating key value areas, including revenue per employee(see attachment to that document please)

Thank you for sharing the information, I have been looking the resolution for the same issue.