There's a guide to prices here:
http://www.croatian-adriatic.eu/istria/croatia -prices.html
To convert kuna to pounds, you should (theoretically, based upon the current exchange rate) divide by 9.1578. As a more practical guide, just divide by 10 and add a small amount (about 10%) on.
e.g. the price of a beer is given as 5 to 8 Kn. Dividing by 10 makes that �0.50 to �0.90. Adding about 10% on gives the price of a beer as roughly between �0.55 and �1.00. (The cheaper price is likely to refer to roughly half a pint of the local draught product, so you could probably buy a 'pint' for not much more than the equivalent of �1)
Chris