To add to the discussion above, the charter market is one that you wouldnt touch with a barge pole from a busines perspective, its very much a niche market and one which very much, and correctly so, comes secondary to the operational railway, but that means that it means that it can massively impact on the relability of the tour promoter.
Roughly you are looking at between 18-20k for a full package of safety case, crew, stock, track access and a basic locomotive. Those costs go astronomical when you start doing midpoint loco changes, using heritage traction etc.
Getting the right supliers here is crticial and forming a mature relationship with them is essential or you'll fail straight off. Within of the world of spot hire TOC's etc theres a number of rather shall we say interesting characters, and getting someone on side from within them is so important, if they dont have respect for you on a business level, they will rip it out of you, and charge you an arm and a leg more than they should.
Then you have your own costs as the promoter, any staff you might need to run your operation if you're one of the big ones, and then theres distributing and manufacture of tickets etc, as well as marketing costs. Putting a tour on a couple of forums is not enough to get bums on seats!