There are many different reasons you might consider an ASO as an administrative solution for your company. After having an expert look at your current situation, there are a few major indicators why you might want to look at an ASO. Here are a few:

Your current costs for benefits, through a PEO, may be more then through open market
Currently your benefits and payroll providers don't communicate so when you term an employee on payroll you also are responsible to notify the benefits carriers. With an ASO, once you term an employee on the payroll, the ASO knows to term the benefits and forward the COBRA notification
You are paying for individual services that don't interact - benefits, payroll, workers' compensation - the ASO manages all these services for you so there is constant communication - the benefit of a one stop shop

If you are currently with a PEO and have been with the PEO for awhile, there is an excellent chance that your cost have "adjusted" and are now substantially higher than you think. The administrative charge the PEO is charging may be clear and reasonable (or may not be clear) but what about the other factors:

Section 125 credit for the EMPLOYER - when you ask many times they say yes we are offering Section 125 credit, but they fail to tell you it's only the EMPLOYEE's credit they are providing and they keep the employer's side as a profit center - for example if your employees pay $100,000 for their own benefits annually, the Section 125 credit for the employer would be as much as $7,650 - right to your company's bottom line.
State Unemployment Tax - The PEO doesn't use your earned tax rate, they can pretty much charge what they want for SUTA since you are paying them a leasing fee not SUTA - in FL, for example, if you were at a 1.02% SUTA with 25 employees and the PEO, without your knowledge, now bundles your SUTA in as part of the total fee at 5.40% that would mean your company is paying $8,760 more than you were.
The PEO can also charge a higher than state rate for workers' compensation coverage
Maybe the first year you were with the PEO, the medical rates seemed great. The market has changed substantially. Medical rates have gone down dramatically in some cases. This is especially true if your group has had a lot of claims. While the PEO adjusts your pricing annually based on your claims and demographic information, a carrier on the open market cannot even ask medical questions for groups under 50. This means there would be absolutely no correlation between what the PEO is charging and what your company's rate might be on the open market

We have many examples where we have saved companies with as few as 20 employees 5 figures in the areas listed above. The only way to do this is to have an expert analyze your company's current situation and create a cost comparison to show you the differences.