The most widely advertised rate in Scotland is £120, so that establishes a benchmark to which I try to adhere. That said, I will occasionally go up to £150; but only for someone who is in some way special, such as a lass who will include anal and/or BDSM in the price.
I would only go over £200 for a punt of two hours but, as has been mentioned, it can be a bit boring. Again, some girls are easy to be with..