ZERO, Do it myself (3x all work fine). Really is made out to be a lot more mysterious than it is. It is time consuming to do it right, but you can buy a extra rear end and work on it without taking your car out of commision, and still it will still be cheaper than the labor you pay someone else. In the end you will have learned a lot.
If you choose to work on the existing axle housing, and you have access to a bearing press it will only cost a few bucks (less than $20) for new pinion bearings (which I would strongly suggest), and thats about it. If you don't have access to a bearing press, have the inner pinion bearing pressed on where you buy the bearing (good Napa shop or simular). They will usualy do it for free or just a couple of bucks at most. BTW, only buy Timkin bearings for this, two reasons: One, they are by far the best. Two, This is waht Ford uses and there are variations in length with other brands, this will effect pinion depth.
First time I did it with a friend to his car, we had a local shop (Randy's Ring and Pinion) to call when we had questions, and even had a tech look at the gear pattern (pictures) for free, he approved of the pattern and sure enough the gears run dead quiet.
If you are installing Ford factory gears it is a slam dunk, no need to measure pinion depth, just measure the shim on the old pinion, and install the same shim amount (or same shim all together) on the new pinion, then all that is left is to set up the backlash and bearing pre-loads. Again this can be made a lot easier with a solid pinion spacer instead of the crush sleave and a old worn pinion nut. Read up on it, Randy's ring and pinion has instruction on thier web site, also several others available out there too.
The money I saved doing the last install was spent on the Eaton Diff.,makes for a great bonus from doing the work youself.