Yes, there's been a number of occasions where the difference has been negligible, where promotions have made the miles earnt worthwhile, of even when it's been cheaper.

I've even flown LHR-BCN in BA C for less than FR were charging for STN-GRO...

LHR-LIS is another example, and LHR-PRG. The latter was an IT fare, a package deal with the premium flight that was less than the airline wanted for the economy flight.

LHR-AMS-BKK was the final example, on KLM. There was a particularly good mileage promotion on at the time and the fare was around £1000. I did the sums, and figured out that, based on comparable economy fares for the dates I needed of £600, the extra £400 was well worth the comfort, and the extra miles would pay for another trip of similar length.