I sometimes use my old SonyEricsson mobile to go online with my laptop. I have a Thinkpad running linux, and access the phone via bluetooth. That works really well: As long as the phone is on, the laptop can use it automatically.
I had to set up the connection manually, but that was quite easy, and is well documented in various places.
The only technical problem is that reception quality varies a lot from place to place, so I sometimes lose signal, particular in moving vehicles. But that would be the same for a data card (maybe worse, because its antenna is close to a EMI-noisy laptop).
As to cost: I have a prepaid plan for my mobile; I pay 0.24 Euro per MB, with no monthly minimum or recurring charge. Fine for me, as I mostly only use it for mail checking and occasional browsing.
I'd recommend you find and use a simple pay-as-you-go plan for a while to see how much you use it. Then take a suitable flat fee plan.