as a retired builder of 28 years, I would need to know more about your plans. for examble: the foundation-slab or crawl space, how will the room be used? as a kitchen, family room or bedroom? will it have a bathroom? how about the siding? is it new enough to match or will you need to reside the whole house? same question about the roof. how many windows and or exterior doors?
Is there easy access to addition area ? can a cement truck get up close or will it have to be wheel barreled?
is the electrical service large enough to feed the addtion?
once these basic questions are answered for me I could give you a very close cost.