thats more or less what happened to my "older" gent - except they started off at £150 and because he was a pensioner, they would reduce it to £140.........then when they tried to set up the charge, a bill was showing up on his screen for £499 - he queried the difference and they brushed him off with something along the line of taxes and charges, but the amount to be taken would still be £140......
sadly, he gave them all his credit details and full access to his PC ........ barclays rang him within minutes and blocked everything, so I don't think they managed to take anything
he said to me that it was in the back of his mind the whole time that it was a scam, but they were just so convincing ..... for nearly 3 hours on the phone