They'll only pay any loss of earnings, you can't turn up as a dole dosser and claim the maximum (around £70) a day! If you run your own business however, and by pure chance hadn't yet completed a tax return to prove income, you can be losing as much money as you like!
They will pay for your travel regardless, and food as said, and if you don't spend your food budget you get the money instead.
After 2 weeks I came out with near enough £800, and apart from the days and days of waiting around to see if you'll be picked I really enjoyed it! So much so, I elected myself as foreman for the second case we did
They gave me the choice to opt out of doing it altogether (long, boring story) but my mum talked me into doing it in the end because she enjoyed it quite a lot too, glad she did!