how much does something like this cost if we're moving from a detached to a attached garage?

The cost will depend on many factors. Adding an attached garage always impacts the existing home to some degree and therefore typically part of a larger project. Materials/details and local market conditions will also affect the cost. I suggest hiring a local architect to help you develop the design and budget.

What if you already have a 1-car garage with little storage above and want to make it into a 2-car garage with a room above? Would that be over $100k?

It might be over $100k but too hard to predict without knowing specifics on existing conditions and local market. Again I would suggest consulting with a local architect.