Yes, it should be cheaper. My phone costs me less than £20 a year because it's the simplest "pay as you go" phone that I could buy, and I use it only for texts and a handful of voice calls. But you have to balance that against what you would lose; for example, not being able to access WP while you are not at home (assuming you have some other way to do it at home otherwise.)
At the moment, you don't have the smart-phone. See how you get along with it for a little while, and if it suits you, stick with it. Otherwise, there's no reason that you can't change your mind, and maybe use some of the other tips that people have given to limit your spending.
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