We were looking at something similar, but in oak and are getting quotes in the same ballpark... If anything, walnut is generally dearer so you are not doing too bad. Where have you looked?
Again, try Heiton Buckleys as well. They have a good range of doors. I got Oak Veneer Doors from them and painted them white.. I wanted a 'heavy' door but also wanted a white door and could not source a quality white door so went with oak and painted it... Cost about €100 each.
If you are in the west try Walsh Timber flooring in Kiltimagh, Co. Mayo. They have a large selection of doors and are quite competitive. No affiliation just a happy customer.