Really what you pay depends on the company you're buying... Usually it's 6 months of income for the provider you're buying out some will try for 12 others will settle for less.
For $1,500 USD you're not going to get much but if you already have infrastructure to move those clients to you can eliminate the provider's operating costs for the most part and make the income near 100% profit.
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Mike has summed it up very well. I believe that you would usually have to times their income by 12 months (annually) but due to the high competition and recession you can get more for your money. This is certainly a good time to buy if you have the funds and time to do so.
As Mike said it greatly depends on the business your buying. If you find a company that is struggling the price may be more negotiable. If you know you can turn this business around it may be a good move.