If you do get one installed, the things that actually vary between quotes:
Cable run length. The distance from your consumer unit to where the charger goes is the biggest driver of price. A charger on the wall next to the fuse box is cheap; one at the far end of a garden needs a lot of armoured cable and trenching.
Whether your consumer unit has capacity and the right protection. An older board sometimes needs work first, and that is where a modest quote becomes a large one. Ask specifically whether the quote assumes any board work.
Whether load management is needed. In homes where the supply cannot comfortably take a full-rate charger plus everything else, a device that throttles charging when the house is drawing heavily is required. Adds cost and it is not optional where it applies.
Earthing arrangements, which sometimes need extra equipment depending on your supply type.
Questions worth asking every installer: what does the price include if the board needs work, is load management included, and what warranty covers the unit versus the installation.
And on the unit itself: tethered cable or a socket is mostly preference; the tethered one is more convenient daily, the socket is tidier and survives you changing car. Neither justifies a large price difference.