We're talking about 5GB/month for €29,-/month (24 months)

The same carrier's offer without the netbook is either
10GB/month for €30,-/month (24 months) or
3GB/month for €20,-/month (24 months)

So it depends... you can do the math like "the netbook-offer is only half of what I get in the regular contract, so I have to double the price each month to find out what the netbook costs me" - but thats nonsens. If you know you don't usually need more than 5GB, it's simply the same price and you get hardware you need instead of GB you don't need.