Expect to pay around $1000.
And i agree with Allcarfan, pulling an engine is not for everyone.
Hell, some shops can have the engine out and back in, before you'd even be able to get the H pipe off.
There is also more peace of mind, when you have a qualified mechanic do your work, espcially in a DD.
My buddy owns a mustang only shop, and i get to see all the butchered home owner work first hand. Sometimes it's not pretty, then the money they saved doing it themselves goes to having him put it together again, this time correctly.
Problems that he sees includes: Loose bolts, wrong bolts in wrong places, bad HG jobs, bad intake gaskets, missing bolts, and the number 1 problem has got to be, screwed up vacumm lines.