Its a difficult question to answer, simply because domain values are highly subjective.
First, you need to value the domain name. Lets say hypothetically, the domain name is valued at $5,000. Thats making the assumption that I would sell it at that price instead of leasing.
If the customer wanted to lease it, I would divide the sales price by 24. Breaking it down over 2 years. This leaves you at $208 per month based on that pricing schematic.
Personally, I wouldn't go less than $200 per month, simply because it becomes a bigger hastle to collect money then its worth for anything less. I would also put an air tight contract in place, that covers all circumstances, its a problem I ran into on a payment plan earlier this year.