You can get doors in steel from a £few hundred but if you wanted a really top quality job in Oak you might be talking as much as £4,000.
This site has prices on Western red cedar doors in a range of styles but I can vouch for them as I've never used them. Prices seem to be typically £1500 to £2000.
http://www.garagedoors4u.co.uk/WoodenSideHingedRange.htmlIt's not something I'd buy online. Best to see the quality first.
You can get them in GRP fake wood for around £1300 (Sorry Wolf).