An excellent point. Although you can get a GPS running watch for about £70 in the UK[1] complete with wrist-based HRM (which is fine for most casual runners, I think.) And that'll last you a good few years, at least.

[1] https://www.decathlon.co.uk/onmove-500-gps-hr-watch-id_85290...

Plus, you absolutely don't need a running watch to run a marathon. That cost estimate is absurd.