My Fitbit was about £70. The Fitbit Zip is a slightly simpler, cheaper model at about £40. I used several Pedometer apps on my phone to see if I would keep up the exercise. The best one was the Runtastic pedometer.
I forgot to say - if you have a FitBit, and you know other people with them, you can link up with them, so you can all encourage each other.
My dsis has one, and has sent me a FitBit friend request - my big concern is that she is so much more fit and active than me, I worry that she will be judging me for how few steps I take - but that is partly my insecurities (only partly - she does have 'form' for being a bit superior).