This sounds like a scam to me. Are you in the UK? If in Scotland, private clamping is illegal as per case of BLACK v CARMICHAEL 1992 which provided that unless a statutory body has clamped you, it would amount to extortion.
In England I don't know, but see this link:
http://www.tradingstandards.gov.uk/hillingdon/ cons9.htm
Certainly contact trading standards to check that the firm is legit. Revisit the site to see if there are signs around (the bogus clampers will often only erect signs for the period of their working the area).
But by the sounds of it, the price going up and only accepting cash would indicate a scam.
It's unfortunate on this occasion if this is the case, however in future, always wait for the police to arrive before paying, and if you can remove the clamp from your car without damaging the clamp, you should do so and then drive away.