Just go to a "Phone House" or any other non carrier branded phone shop and you'll be able to get a prepaid normal SIM card and they can "cut" it into iPhone size.
Or I think you get a iphone sim at pre-paid at any Vodafone shop.

With a Vodafone pre-paid sim you can activate your "Blox", just ask in the shop. Rates are as follows, use Google translate: http://www.vodafone.nl/internet/inte...lefoon/kosten/

Web Blox: 9,50€ per month (you can turn this off when you leave, or only load 9,50 on the sim) 200mb Web+SMS blox: 14:50 per month 500 mb
Web+Sms+Bel BloX: 34,50 per month 1000mb
Or for 2€ a day you get 20mb per day.

Edit, no idea about rates in Belgium.