Dry first, then finish
We've restored a lot of flooded finished basements. The ones that stay dry have a few things in common: no foundation leaks, a sump pump or backwater valve where needed, downspouts carried away from the house, and insulation details that don't trap moisture. We check all of it before we frame a wall, and any fix is its own line on your quote.
Legal secondary suites
A legal basement apartment needs a building permit and has to meet Ontario Building Code requirements, including fire separation, smoke and carbon monoxide alarms, egress windows, ceiling height and separate heating or ventilation in many cases. Each requirement goes on your quote separately. You see exactly what legal costs, and nothing gets skipped.
Many GTA cities pay part of the cost of flood protection like backwater valves and sump pumps. If you're finishing a basement, it's the best time to add them.