As a greater number of people unlock their phones and make them usable on multiple networks there are a greater number of pay monthly Sim only plans available. These use pay monthly SIM cards and generally last either thirty days or 365 days although some networks also offer 18 month pay monthly SIM only plans for you to look at.
Longer term contracts are still more cost effective than plans where you are paying for the phone as well.
They usually offer you more in terms of airtime and data use. You may find that a single network will provide you with several options where you can choose more air time at the cost of data usage and vice versa.
Some networks will also allow you to choose a very low priced plan that may just cover texting or data usage. This can be helpful if you know that this is all you will be using your phone for. It is always totally up to you how much you will spend and what you want the terms of your contract to be.
If you know that you will be texting quite a bit with your phone you need to make sure that the pay monthly SIM cards you choose also have unlimited texting. Sending and receiving text messages can add up quickly so it is important that you make sure you have enough coverage to keep your costs as low as possible.