What this trip costs
Costs split into five categories below. Your total depends on which option you pick in each one, not on one headline number.
Accommodation: the single biggest swing factor
A shared hostel bed or a basic B&B in the smaller villages costs considerably less per night than a hotel in Carlisle or Newcastle at either end — and since the smallest stops (Once Brewed, Gilsland) have the fewest choices, price and availability both tighten in peak summer months. Budgeting per-night accommodation as your largest single line item, and shopping it around before committing to a full six-night plan, is the highest-leverage way to control total cost.
Baggage transfer and full packages
A baggage-only service moving one bag between six stops costs meaningfully less than a full self-guided package that also books your accommodation and provides route notes — and a fully guided trip with a leader costs more again. Decide which tier from self-guided vs. guided you want before comparing quotes, since the categories aren't really substitutes for each other at the same price point.
Fort entry fees
The trail itself is free, but Chesters, Housesteads, Birdoswald, and Vindolanda each charge their own separate entry fee as heritage sites in their own right — Chesters, Housesteads, and Birdoswald are managed by English Heritage, and members get free entry, which is worth pricing against several separate day-ticket purchases if you're visiting more than two or three sites. Vindolanda is run by an independent charitable trust with its own separate ticket, not covered by English Heritage membership.
Transport to and from the trail
Rail into Newcastle at the eastern end is generally cheaper and more frequent than the equivalent trip to Carlisle for the western end, particularly if you're travelling from London or Scotland. Add the cost of a local bus or taxi from Carlisle out to Bowness-on-Solway itself if you're finishing (or starting) there, since it has no railway station of its own — see getting there.
Food and incidentals
Pub dinners and packed lunches along the route are the trail's normal daily food pattern rather than self-catering, since most overnight stops are B&Bs and guesthouses without kitchens — budget accordingly rather than assuming a self-catering-style daily food cost.