I purchased an iPhone from o2 on pay-as-you-go a few months back for around £350 i think it was then. This included a new sim-card that enables free 3G data usage and free wifi at no extra cost. The guy even changed my current number over onto the new sim card while I was there in the store.
Realistically you shouldn't need to purchase a further bolt-on for the web access as it comes as standard. Check to make sure you get the free data package first of course though.