SMS packages are written in credits. 1 credit is not necessarily 1 sms, because prices for sms differ from country to country and depend on the usd rate exchange; that is why we use credit system. Usually, 1 sms can vary from 0.5 credits to 1.5 credit depending on the country. You can check the prices for sms packages in your admin interface, in Plans&Prices//Additional items//sms packs.
And here you can see approximate prices for sms credits: