The art board size for an image as large as you mentioned isn't typically built at the size that it will output. Instead, it's usually built at a fraction of it's actual output size, like maybe only a...
In any business hourly rate can make it or break it
Do you find presenting your hourly rate to a client can put your client in judgmental state, cause all they see is $$ signs and think
they can get it cheaper somewhere else? Now all they think is so many hours = $$$$
This is after the cost of the project scope was given and more work was required and charged by the hour.