FFS you didn't get shafted! You got many quotes, they were all the same price, that is what is know as THE GOING RATE FOR THE WORK YOU REQUIRE IN YOUR AREA. It's not bleedin' rocket science, and even by your own admission you'd have been happy to pay £300 for the work just as long as he took two days to do it. That's utterly moronic, and there's no way in hell I believe you for one second when you say that you'd have paid him more if he'd have taken longer. No-one would. The price is the price. You may slip him and extra tenner, but you wouldn't give him another £150.
Next time, I suggest you just do it yourself if you don't like the price quoted. If you didn't like it, then more fool you for accepting it.