Some say 4 or 5 times the NET turnover, others say 2 or 3 times the annual net profit.
Make sure your accounts are well and truley up to date, and signed off by an accountant, you will have to prove your turnover and net profit. I bought a small cleaning business to bolt onto our current business some 4 years ago, nearly got bitten, but took some good advice beforehand.
You might have a figure in your head that you want for it considering how much you have put into building up the business. The potential buyer will want cast iron guareentees about clients etc, plus added value items such as equipment, computors, they will want to see your accounts in full.
Just my thoughts