As much money as a normal smaller laptop. You didn't reveal which old Sony you have so I can't tell if the slow netbooks would be acceptable.
I suggest you rethink this and no one I knows would use a netbook for long periods.
My Sony laptop isn't working so I am thinking about getting a Netbook. I like the idea of having less strain on my shoulders and the smaller size PC. I already have an external DVD/CD burner and an external Hard drive. What's the best Netbook on the market? I am looking for good performance and good battery life. Is there any netbook that comes with the full Windows 7 program that does not expire (not a trial)?