Coffee/Tea Mixup

Richard Wiseman offered up another nice little 'Friday Puzzle' for the weekend, over at his blog:

http://tinyurl.com/39f59ej

For those of us on this side of the pond, I'd substitute 50 cents for his 50 pence, in which case I've given the answer below (I'll let him explain the reasoning on Monday --- there's actually some slight ambiguity in the problem as stated, and depending on the potential interpretation my given answer below would be different):

ADDENDUM: I originally stated a WRONG answer below (now corrected) --- Wiseman's explanation here:

http://tinyurl.com/22k4mpv
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answer: $1.00 50 cents!