Twinklestar here is a link to Dandy's which sell top soil and compost. It will give you an idea of the prices. You could incorporate spent mushroom compost into the top soil or make your own leaf mould this autumn which could be dug in to future raised beds next year. This would bulk out the top soil. When buying top soil try to buy sceened top soil as this has all the rubbish and stones removed.
http://www.dandys.org/prod.asp?categ...-Manure&CID=50