What’s peoples experience of how often these need replacing… And any idea of the usual price?

Can’t comment on the how often, as it depend on how often you ride, but off-hand they should last years between replacements (mine did, 26er v2?). As for price, if you buying new, then jungle will replace them FOC, but off-hand iirc they weren’t a weird size, so a £10 each in the worst case…