This is a pretty concise overview on the cost of payroll services for small businesses. 

http://www.choicevendor.com/blog/2010/03/how-much-does-payroll-cost-we-compare-4-top-providers/comment-page-1/#comment-1129

Of course, if you bundle in additional HR services like software, benefits, compliance and workers comp, there are greater savings to be seen.  That is why for small businesses, we recommend PEO and other total HR outsourcing solutions to gain economies of scale so the payroll is basically free.