Likely it will be higher than you think but 15k is excessive. I would say tub cost + 1 day labor for 2 people + disposal. Depending on the tub 5-8k total? You'll likely pay for a full day on the labor even if it takes less just because it's inefficient to pull people between jobs in the same day.

Never work with someone who is pressuring you to sign that day. They should give you a quote and a number of days that it's good for. Either they didn't want the job or they prey on ingnorance.