For a normal door, I think the price a couple years ago used be around 1.2k-1.5k for a composite.
But I think you should try to see where the draught is coming from in case it's not just the door, check for example the bottom of your skirting, fuse box, alarm box, letterbox.
If you find problem areas you might be able to improve things with some airtight tape eg.
https://www.airstop.ie/Airstop_Elasto_T ... 52419.aspx. (Be careful with this stuff it's very sticky. Also it's relatively expensive so people tend to not put it everywhere they should.)
Hallways are a problem area, it's where most services come into the house so you've had people making many holes in the wall both on the inside and outside.