Really? I don't mean to sound like a smart ..s but that's like asking how much a car costs per pound, I guess you could come up with an answer but.....

In terms of difficulty, it's really not much different than building a traditional covered porch, just with the expense of deck and rails over your roofing materials or under-deck roof system. But a back porch can vary wildly in cost based on size, height above grade, attachment to the house, flooring material, trim (is it trimmed with finish wood or composite trim, or is it a "chicken coop"?), electrical, etc, etc. I've done $20k porches and I've done $200k porches. Your best bet is to talk to a local pro.

well it is not like building a back porch the roof of the porch now has to built to a standard for a roof top terrace able to hold furniture, people safe railing to code all things you need to know before any one can answer the question about how much. You also need to provide a lot more info starting with pictures and the reason for the deck in that space.

Get local estimates. I lived in San Diego and then moved to an Amish town in Central Pennsylvania. Prices are very, very different, and not always less in Amish country, but always very different. I can't guess on the price of anything here.