Considering a home addition

Depending on location, finish materials selection, and mechanicals (Kitchen, bath, etc) you will be looking at between $65 - $200 per sq. foot. of new space. 30*24 = 720

My guess is you will be somewhere in between to have your addition built.

stone and brick cost more than vinyl siding tile cost more than carpet for floors faux painting costs more than builder white everywhere kitchens and baths cost more than no kitchens and baths basement costs more than slab slate roof costs more than 3 tab shingles

There are lots of variables. No one here can tell you for sure. We can tell you that your approach to figuring a price will not work well.

Also, dont forget the unforseen. What if when you prep you existing structure to recieve the new and you discover that the foundation is crumbling? Will current elec service, septic, water etc handle your expansion?

You can ask some folks that have done additions that you like, and are similar to what you envision, how much they spent per sq ft. Use that as a ballpark number to see if you are pricing yourself out of the neighborhood.

I think that the $65.00 a foot is a little low. Since the war concrete, lumber and plywood have jumped 20-40% depending on where you get the stuff. A friend wanted to pour an simple pad in his back yard. Cheapest concrete he could find was HD @ $136.00 a yard used to be $85.00

You forgot the stem wall, insulation, windows, wall board, electrical, plumbing?, paint, exterior siding and the list goes on. Results will vary depending on where you live

if you bring in some sort of a plan to home depot they would give you an exact dollar figure pending fluctuating building material cost.