DTG don't change their prices, so likely to be £25 (or whatever a normal route is in the currency you use)
If it’s a solid release with a nice timetable then I wouldn’t be opposed to pay more. Say £30. But I would expect there to be few bugs and any that do get through to be patched soon.
Spirit Of Steam: Liverpool - Crewe has just gone up on steam for pre-order and appears to be the same price as a normal TSW2 route at £24.99. Interest, as part of a bundle that includes the game as well: https://store.steampowered.com/sub/525262/
So some more info that has just come through on steam, the price of the route is confirmed at £24.99. Plus, there's also a TSW2/Spirit of Steam starter pack for anyone who doesn't have TSW2 and also wants the new steam route, which is priced at £29.99.