I believe it depends on what handset you go for under the chosen tariff.
You wont get any additional costs on top of the "per month" tariff itself ( unless you go over your allocated web browsing allowance, but you would need to be downloading a hell of a lot to do so

).
I use mine hooked up to my laptop quite a bit and I've yet to be anywhere
near my download limits for browsing the web.
The Vario II I went for was advertised online at £69.99.
I think we printed out the online price and got it around £10-15 cheaper than it was instore.
I would recommend a Vario II as its a nice wee handset. I was wavering between it, a Nokia N95 and a Blackberry, but the fella in the store knew his stuff and suggested I went for the Vario II.
The prices for the Vario II might well be lower now since the Kaiser/Vario III has appeared, would be worth checking.
Also, dont want to appear to be a T-Mobile "fan-boi", but I've had not a bit of hassle with them since I signed the contract. Their Tech Support is excellent and most of it is based in Scotland so if you have any probs and call them, your not just getting shunted through to some call center abroad.
Are you in a 3G area?. I have found mine does well on 3G but HSDPA access seems to be pretty confined to larger towns and cities. Mind you, I'm up here north of the border where its cold....lol.
The 3G speeds seem pretty good enough for me anyway, its just a bonus when you enter an HSDPA area.