It really depends on quality of the names in question : ) But, as a general rule, try to figure out your desired price, and make sure it is somewhat realistic. Then, take this price, and divide it by some factor, lays say 60.
So if it seems that your domain's worth is around USD 6000, then you can rent it for 5 years for USD 100/month.
Of course, there are many other factors, most important one is : does it make sense for end user to rent your domain, instead of purchasing it? You need to exactly understand all fundamentals behind this question.At the end, it is all about agreement.