Nobody can price an addition over the phone, but anyone can explain what the price is made of. An addition is a small house attached to a bigger one, so it carries every cost a house carries: excavation, footings and foundation, framing, roofing, siding, windows, electrical, plumbing, HVAC, insulation, drywall, flooring, trim, and paint. What changes the total is which of those are simple on your particular lot and which are not.
The three line items that move the number most are almost always foundation, roof tie-in, and mechanicals. A flat, dry lot with easy access is a very different excavation than a sloped backyard a machine cannot reach. Tying a new roof into an existing one is straightforward on a simple gable and expensive on a house with three intersecting rooflines. And if the new space needs its own heat, or the panel is already full, that is real money that has nothing to do with square footage.
The two places budgets go wrong are allowances and existing conditions. An allowance is a placeholder for something you have not chosen yet, like tile or fixtures, and a low allowance makes an estimate look competitive while guaranteeing an overage later. Ask what the allowance covers and what the thing you actually want costs. Existing conditions are what the house has been hiding: undersized framing, old wiring, a foundation that is not what the plans assumed. A contractor who has opened up a lot of houses in this area will put a realistic number against that risk instead of pretending it is zero.
Set aside ten to fifteen percent beyond the contract price and treat it as insurance rather than spending money. On a newer house you may not touch it. On a Trenton rowhome or a 1950s Levittown model that has been expanded twice already, plan on using some of it, and be glad it was there.