I'd say go to a good optician and on their contact lens scheme for the first 12 months. It might cost you a bit extra but you'll have the benefit of their support and they usually provide extra lenses at no charge if you damage the ones you have. If you're on monthly lenses the chances are you'll need at least a couple of extra lenses in a 6 month period and maybe more if on dailies as they tear easily.
You would need to factor that into the cost of online lenses. I believe that it's illegal to supply lenses without a prescription so you'll obviously need to visit an optician anyway. Plus it's usually free eye-tests if you're on a scheme as well as well so it's not always cheaper to buy on-line.
The cost will vary depending on what you have - daily, monthly, standard, varifocal, tinted, etc - and you need the right solution as well.