I have had good experience with AON, which did find some surprises on an executive we checked out. Although the surprises were unknown and had not previously been disclosed, they were not deal-breakers. Very pleased with their work. From memory, invoice for 2 in-depth checks was $1-$5k.
Agree with comments from @zippy and Michael's:
Get references and check them out. Call the references and ask them questions that give you the information you need. i.e. What is their time keeping like, What is their sickness record like, describe the quality of their work etc. Try and keep the questions open, so that they don't answer with a yes or no. Get them to talk about them rather than reply with a short yes or no.
In addition, if you have narrowed it down and to one or two then take them out for dinner. Get to know them and their background. This way you get to know them on a personal level instead of in a formal interview.
If you gut tells you no, then go with it. Don't hire anyone who you may feel unsure of.