Surely paying £50 instead of the other 750 Air Miles is actually the standard cash for miles substitution rate (eg Gatwick to Marseille) but its just that the cash fare you are saving by paying the £50 is higher than it is on quite a few other 1500 Air Miles routes.

Air Miles flights on LCY to PMI are available on dates where either the outbound and return fare are both £69 or where it costs £79 in one direction and £69 in the other. So for £50 you potentially save yourself a cash fare value of £74 (half of £148). The £20 + 1130 Air Miles is therefore theoretically even better value.

However I would agree that the full AirMiles to part cash fare trade off rates are less attractive long haul, for instance on departures to most destinations in the USA.