A power meter is considerably more than 200 quid, so a good TrainerRoad approved turbo would be the best/cheapest way to get a figure to work from.
Unless you’re really into racing then the TR power value, even if incorrect, will be just as an effective training tool than a “real” power number. (Provided you keep the setup consistent).
However you can’t ride a turbo on the road/trails so if you’re a stats kinda person and need those numbers for every pedal stroke you turn then there’s not much else to do than payout for a power meter.