Do some research (you’ve made a start), get at least 3 quotes but preferably more & let everyone know they are in competition with others. I’ve not worked out if your quote is “reasonable” but, FCS, he’s a builder not a charity 
He has to make a living & will quote/charge you whatever he thinks he can get away with (a reflection on your skills perhaps!) to offset being screwed down on jobbing contracts & those that “know”. Unfortunately it’s a wicked world & the fittest will survive, just make sure you’re included & not just paying for it but your making big mistake if just looking at materials costs; that's purely DIY!
You generally get what you pay for but you don’t always have to pay as much as you would think.
He has to make a living & will quote/charge you whatever he thinks he can get away with (a reflection on your skills perhaps!) to offset being screwed down on jobbing contracts & those that “know”. Unfortunately it’s a wicked world & the fittest will survive, just make sure you’re included & not just paying for it but your making big mistake if just looking at materials costs; that's purely DIY!You generally get what you pay for but you don’t always have to pay as much as you would think.