if you've had an extension done, about how much did it cost?

we're thinking of squaring off the back of our house (right now we have a breakfast nook that pops out of the back of our house, and adding about 3 feet to the end of the breakfast nook..

so the whole extension would be 3 feet out from the breakfast nook, and then about 9 feet out from the rest of the back of the house

we would only do the first floor, not the second (as it is, the nook juts out so we would just square off the bottom)

how much would something like this cost? who do we call for estimates?

eta, it would be a total of an added 175ish sq ft