In my opinion, you may try and arrive at this figure using the "present value of future money" method. This would be a simple calculation. Through this calculation you will determine the value of money TODAY of the series of payments that you would have otherwise received over a period of time.
Here, if you are due to receive, say $ 1,000 Per Month for next Ten years, the present value of money will help you arrive at a figure which you can receive as a lump sum TODAY and forego the future money. I think, this is what you are trying to work out.
The following link will help you in understanding this concept http://www.darwinsmoney.com/present-value-of-money-explained/
The method of calculating is given at http://www.arachnoid.com/lutusp/finance.html . Just put in your figures and it will help you at a fair value of the receivables that you have over the period of years TODAY.
Don't hesitate to come up for further clarifications / assistance.
I am sure this would help...