Easy, get another plumber outside. Bring to his house. Show the job done. Ask if such work how much.
Don't pay yet. Ask for invoice with the breakdown of charges. Labour work price for that x hour of work. Material cost charges. When u have that, u can see what is causing the huge bump. If he don't allow u bring another plumber to check how much for such work, don't pay. If u don't pay he can't do anything but to bring u to court. In court u at least have the invoice break down to show scam. And I can guarantee he most likely won't go that way. Because it will involve the plumber and income tax. If he can charge such work for 2k,which u say is easy work (your word against his), he sure is a rich plumber.

Definitely scamming u if another plumber charges 750 by looking at pic. Even lets say 1k also you are overcharged extra 1k. Good luck to you.