Prices are a lot higher here than in the UK so I did quite a bit of research before finally deciding on Virgin. I already had an old iPhone 4 so all I needed was a sim. So I bought a deal for $20.000 per month and I get 10 texts, 50 minutes, 350MB data and unlimited Whatsapp. I have wifi at home and at work so I don't need that much data, and I mostly use my phone to Whatsapp people here or friends back home. It is certainly not as cheap as my old contract in the UK, but it is the best I found here. It is a sort of contract but there is no direct debit so you have to go and pay it each month at a supermarket or you can't use it.
Click here to see all of Virgin's plans.
IMPORTANT EDIT: I have just seen (22nd March) a new company ETB has released some very good plans. I will be changing mine to the $30.000 per month with 1GB, 150 minutes and free unlimited Facebook, Whatsapp and Email. Way better value than Virgin. See here.