Selecting the contractor because they provided the lower cost doesn’t mean that is the real cost. I have seen this numerous times over the years, clients pick the lower cost.
Now what?
The number was wrong, the contractor did not include 1/2 the work and you find out when you are to far in. You signed a contract that has no finish details but pages of how you cannot give a bad review or negative press.
If you have no details explaining what is included in your project then do not hire the contractor, this is true for Plumbing, Electrical, HVAC, Roofing, or General Contracting, Lawn Mowing, Car Repair, etc.