i dont think you can buy them brand new just on their own or pay as you go.
they are only available through contract.
looking on ebay they go for anything between £200-£400
i put it on ebay yesterday:
blackberry storm 9500 in perfect condition on eBay, also, Mobile Phones, Mobile Home Phones (end time 28-Mar-09 16:11:46 GMT)
but if anyone wants to make an offer as near to £300 then i will remove it.