It's best to match your mobile phone cost with what you can afford and how much you need to use it.
If you're tossing up between a pre-paid or a post-paid plan, head to this site to see the pros and cons of each
Generally, pre-paid deals allow you to have more control over how much you spend because once your credit runs out, you can't make outgoing calls or send texts on your phone unless you pay more money. On the other hand, if you sign a post-paid contract you will have to pay excess charges once you go over your data usage and call or text limit.
If you want to compare plans, the website Whistleout is a good place to start.