They're certainly not ripping you off at that price. You'll do well to get it significantly cheaper. Sounds a very fair price. Assume maybe £350 for the door, £180 for the window and £80 for the toplight. Add the cost of fixings, sealants, trims etc, making good inside might need plaster too. It's not a hugely complicated job to do, but for a couple of hundred quid of fitting charge, not worth the hassle of tooling up and learning to do yourself, unless you were happy to do a houseful. Then these days, unless you get them fitted by a Fensa firm, you need the local Council bod to sign it off, and take a big fee. This is assuming remove and refit new, rather than straight fit into something like a new porch etc, in which case your price might be a tad high.