Cost really depends on the vehicle. Every aftermarket service contract will classify vehicles into different categories based on "potential risk" factors. Generally, this works like any type of insurance. Considerations include: age, miles, make, model, diesel/gas and so forth.
If you are purchasing a relatively dependable make 2-4 years old with miles in the 20-40k range, I would expect to pay somwhere bt 1500 and 1900 bucks for a pretty inclusive coverage.
Make sure you know your deductible.
National Auto Care or NAC for short is a company with a good reputation locally.
Good luck with your sale!