Duncan;
All of the advice posted here seems appropriate, but nobody addressed your question about cost.
Regarding a new mast, you should contact any of the mast manufacturers listed and talk with them. When I was shopping for my new mast last year, I found all of them VERY helpful. There is a lot of real engineering involved, and you will have many options which will effect the cost considerably. Options will include running and standing rigging, wiring, lighting, number of mast exits, type of hardware, tracks, cars, blocks, toggles, and other little things. Then there are upgrades to consider: an updated boom with internal outhaul and reefing, headsail furlers, vangs, etc. Another factor to consider is that if the mast manufacturer is not local, it may be difficult to buy directly from them. I involved my yard from the beginning. It added cost, but they added a lot of value. I also got a lot of help from Seth (my sailmaker).
For a boat like yours (and mine), the cost for a new spar can range from a few thousand for the basics to many teens of thousands for a complete system.
Also, you must consider all of the issues mentioned with a used spar - as well as the condition of the spar. If you need to replace all the hardware and rigging, the cost of the finished tube section is almost inconsequential. I would expect you could buy a used spar for hundreds of dollars, but don't underestimate the engineering effort and manual labor required to adapt it. Some of the critical issues involve the angles that the spreaders make relative to the mast and the shrouds and the locations of the chainplates.
Replacing my mast and rigging was not an easy process - it required excellent work done by Ballanger Spars, my Yacht Yard in Chicago (Crowley's), help form my sailmaker, and many, many hours of my own effort. My expense was toward the higher end of the above cost spectrum, but I got an excellent result.
Good luck with your project.
Regards;
Steve