At my last assignment, I was asked to compare 3 vendors and make a commercial off-the-shelf (COTS) product recommendation to the client. The client knew their budget and products they wanted evaluated. They didn't know if the product capabilities were all marketing hype or if the products would indeed meet their needs. When doing a product evaluation, I tell the customer they must help me do the following  List the specific business requirement(s) that must be satisfied by the COTS package. List the specific business data or information requirement(s) that the COTS package will need to support.  List the strategic and performance plans that must be met by the COTS package.  List the practices and processes the COTS package will compliment.
By detailing the information above, it demonstrates the requester thoroughly understands their needs.
The customer must then answer 2 critical questions that will impact the total cost of the implementation:
 Does the COTS package need to be modified to work with current practices and processes?
 Do practices and processes have to be modified to work with the COTS package?
Some companies believe if you throw enough money at something, you can fix a problem. Spending a lot of money on a product to "fix" a bad business process just means you spent a lot of money and still have a bad business process.
The last thing you do is create a capability matrix to do a side-by-side comparison of products. Don't paint yourself into a corner! Make sure you complete the Package Evaluation first so the details are available for others to review later. Give your stakeholders the facts. Enjoy this free copy of my COTS Package Evaluation Template.