gear size depends on A) what you use the car for, and B) transmission.
If you use it as a daily driver and still need the best gas mileage for power go with 3.55's ... there is a noticable difference in power between the two but not so much thats its worth the gas money at todays prices. also Id reccomend 3.55's for 5-spds.. but if you run it as a racer or track car, 3.73's or 4.10's
a good set of gears (Ford Racing) generally range from $160-180, depending on the supplyer. I got myne from
www.Summit.com. I went with the Ratech installation kit. Everything except wheel bearings and seals comes with it. or you can get the deluxe kit and get those with it.
labor is up to the installer, i hear some say $200 but around here the only two guyz i trust to do it but I still got hosed on it. $400 so... it was quite expensive but im glad its over with. your car isnt the same without it...