MTBing can certainly be quite expensive, especially if you need to travel to the trails. Depending on what sort of riding you’re doing; it can be quite easy to wreck several hundred pounds worth of gear if you cock something up. OTOH, about seven or so years ago, the only MTBing I really did was round the Grizedale area, so fairly gentle stuff, and it was within riding distance, so it cost nowt really.
These days I don’t mtb at all; my current road bike (only worth a few hundred pounds) I built on March and since then the only thing I’ve changed on it has been a chain and the pedals. It rides great and should probably just crack on with it, but I ordered a custom disc braked frame in February and have been buying bits to build it up with, so it’s been an expensive year really.