I just saw an article on T-zine on Belgian mobile internet prices and I didn't want to keep it from you guys!
To give you guys an insight in the prices of the 3 major operators in Belgium.

Proximus
here you have 3 options.
Access Time Option (standaard) € 0,4158 / 5 min (€ 4,99 / uur)
Access Volume Option € 0,50 / 100 KB
Bundel 50 MB € 15 / maand for 50 MB.
After 50 MB you'll pay € 0,90 / MB*.

I find this a bit expensive actually. Especially with a maximum of 50 mb. If you start downloading content regularly on your mobile phone this just isn't going to cut it.

Mobistar
Until recently I could only find data plans for mobile laptops. In the end you still used a sim card and you could put that sim card in your phone to surf the internet, but as far as I know you couldn't make a phone call with that sim card.
Now Mobistar has a new dataplan aspecially for the iPhone. But my guess is you can just use those subscribtions on your other GPRS enabled phones aswell.
30 Euro for 3 hours calling or 300 text messages and 200 mb of data
45 Euro for 6 hours calling or 600 text messages and 500 mb of data
60 Euro for 9 hours calling or 900 text messages and 1 GB of data

Base
if you surf very irregularly you pay 2,5 Euro for 1 mb
5 Euro for 10 mb
10 Euro for 200 mb
25 Euro for 2 GB
The first three are available in prepaid the last only if you have a subscription.

My personal favorit is Base because of a fair prepaid offer of 200mb for 10 Euro. I especially like the idea of freedom of choice. If I don't want to spend any mony on phonecalls or mobile internet next month I can do that. Mobistar and Proximus do both allow faster connections, but that's only if you are in an area where UMTS is supported.See here on this image for mobistar! and here for Proximus!
Probably the real battle for cheap mobile data plans is about to start, but the above will give you an idea of what you can use. You should also take in mind Mobistar and Proximus both have a dataplan for mobile laptops which are cheaper than their mobile phone dataplans. The only disadvantage you'll have is that you probably won't be able to phone using those simcards.