A professional Pulse-Oximeter as used by the medical profession is likely to be £150 - £200 or more.
Anything cheaper is likely to be inaccurate, or unable to compensate for patient movement or circulatory variations.
A lot of the cost in professional medical kit seems to be a premium added because it is certified for medical use and to cover all which goes with that with a small market.
Non-medical equipment may be less accurate, but that's not necessarily inaccurate to any appreciable degree outside a clinical environment. My GP and healthcare workers all seem to find using equipment one can buy off the shelves in a high-street chemist acceptable enough.
They are good enough for most cases, don't need to be perfect, just good enough to indicate when things are amiss which should then be checked out more accurately.