depends on what you are adding... 

kitchens and bathrooms can range from 110 - 200+

an unheated storage space with a slab foundation might be as low as $50

you should contact a few builders in your area and give them a general idea of what you are looking for and ask for "ball park" estimates. Then you can decide if it makes sense to even consider going down this path...

it might be cheaper to sell and move instead of adding on.

Different builders will probably have different estimates. I would ask around first before making a commitment. If you have a set about to spend on the addition, keep a mental note as to which builder comes closest to your budget. If nothing comes close enough, then go for other options. Just like the previous poster said, moving to a different house can be an option as well.