No Pearls in cracked Oysters!!!
London Underground caps the charges on an Oyster card daily depending on the zones covered:
So lets consider the value of cracking a card. The only criminal benefit comes from copying the card and then selling it. So the average maximum value a cloned card could be used for per day would be around 9 or 10 pounds depending on the zones that the original was valid for.
Now assuming that the TfL network can only detect duplicate cards in the system and then blacklist them overnight, then the maximum value of a cloned card would indeed be about 10 pounds (but on average a lot lower)
Counterfeit goods typically sell for a lot less than the real thing so lets be generous and say the typical cloned card could be "sold" for 5 pounds (again best case)
In volumes less than 100,000 Mifare cards cost about 2 pounds each. Which gives us a whopping 3 pounds margin per card. Plus the cost of a scanner/cracker/copier (say 500 pounds) (and a laptop) then our criminal mastermind has to clone, copy and sell nearly 200 cards before he starts to make a profit.
This is best case - in order for the cards to be sold for their maximum value they need to be sold at the morning rush hour and assuming there is actually a market for these cards and they don't get arrested in the mean time, then you could imaging then making about a few hundred pounds a day.
But in fact Tube station gates are smart enough to stop the same card being used twice in the station without it being used to exit first. So in fact cloned cards could only be used at a station different from the one where the genuine card was actually used.
I'm sure an enterprising criminal capable of executing such a business would be better off contracting to an IT company in the City and surfing Youtube in an office, rather than getting cold and wet and risking prison outside an actual Tube station.