I'm using my N900 most of the time as a wifi device. Most of the time, I am either at home or at the university and I have wifi in both locations.

However, I wouldn't buy a wifi-only n900. I have to carry a mobile phone with me anyway, and I'd rather not carry with me more than one device in my pockets.

The cost of adding 3g to the device is less than you think. It probably adds only few dollars to the manufacturing price of the device. Most of the price you pay covers R&D, marketing etc., and not manufacturing. You want a cheaper device? Do like I did and buy a refurbished unit for less than 350$.

I would consider a wifi-only device, OTOH, if it had a large screen (5''-7'') and higher resolution so I could use it as an eBook reader. Anything that goes into my pocket should be able to function as a mobile phone.