I suppose it depends on how it runs,and how thorough you want to be.No set price.
Replacing the spark plugs is #1 on the agenda,not a big labor or parts cost-that alone,should cure a lot of running problems.
Going deeper,you can change air and fuel filters,spark plug wires,pcv valve,etc.
If you want to keep the car,you wouldn't be wasting money to service the transmission,motor oil change and filter,change brake and power steering fluid and antifreeze.
That's a valid response as well.I went through all sorts of typical tune up and repair items on my 2002 Taurus that had an unusual running problem,including upper and lower manifold gaskets,before I finally came to the cause; a bad coil.
If you provide more info you might get the right answers you want. How many miles,what have you already done,and is your check engine light on? When I say what have you done, I mean what have you replaced up to this point.