How Much Should You Pay For Accounts

[DUALLY DRIVER]

Thinking of expanding into Lawn Maintenance with the economy the way it is in Arizona. How much should I pay for a Lawn Customer that pays 50.00 Bi-Weekly or 100.00 Monthly ?? Is there a formula that puts you close. I called one guy who has an ad on Craigs List and he said he has 450 customers and that he wanted 800.00 each. Sounds very high to me. Any help that anyone can give me would be greatly app.
Thanx,
Charlie