If the exercise was designed to raise the profile of Morgan, then, even if one new person is aware of the brand, I'd suggest it was a success.
Lets just suppose that 10 extra people did become aware of the brand as a result. Lets suppose that 10% of them ie one person decided to buy a Mogan trad. Probably a bit optimistic since the race car doesnt bear any relationship to the road car at any level, but lets assume it anyway. Lets further suppose that MMC make a contribution about variable production cost on a trad of £5000, then that is the value of the sponsorship. Now how much did the sponsorship cost? £500k?
Very rough figures indeed but presumably someone at MMC knows the real numbers. My suspicion is that the return on the sponsorship is a bit like the return to the UK on the Olympics - dreamed up in the mind of someone looking for a reason for doing something he already has decided to do.
But then MMC is someone's personal toy so why not go to le Mans if he wants to do?