$700 is actually not bad considering what is involved with the install. You may be able to have it done cheeper for around $500) but that's about it. Remove wheels, remove brakes, pull plate, remove driveshaft, pull axles, pull gears, remove dif. pinion, install gears, adjust gears, then reinstall everything, calibrate car to new gears, test.
Quite involved...and is highly recommended that it be done by a shop. Not something you want to tackle in the driveway unless you know what you are doing. I know of a couple of guys that did the install on thier own and ended up taking it to a shop because once they finished the install and thought they had it all aligned they got the dreaded rear diff. wine.
in all honesty I'd just go to Orchard ford in kelowna (they are also a roush/FRPP dealer) for that one.. Kaisoboy is right.. there is a heck of a lot involved with doing gears. but IMO, to make it truely worth it get a t-loc while your 8.8" is apart. unless you have one already
Thanks guys, appreciate the info and yes I'm coming to the sad realization it isn't such a bad deal...definitely not a driveway project (for me anyway). Once again this proves to be the best Canadian Mustang board hands down