How longs a bit of string?
First off because of the nature of the web you will be able to access anything they have previously done, so ask to be pointed to sites they have worked on in the past.
Unfortunately because it's easy to set up a web buisness there are plenty of people around, some good and some total rubbish.
As for rates of pay, totally depends on what you want. I have worked on sites which cost �60,000 and ones that cost �100.
I think the best thing to do would be to look at a few sites which are similar in scale to what you want, find out who built them and ask them roughly how much something similar would cost.