Optus is set to hike its mobile plan prices next month, but there is a way around it – at least for the next 12 months. Better still, you can lock in a discounted monthly rate for the entire year, along with some bonus monthly data thrown-in for good measure. Currently, Optus has slashed prices on two of its best SIM-only mobile plans by AU$15 p/m for the entire first year, which'll save you AU$180 in total. That's already a great discount, but the telco is also providing 50GB of bonus data per month. Those are great prices by Optus standards, though it is worth noting that Vodafone has currently discounted its SIM-only plans by 50% for the first 6 months, offering slightly better value over the course of a year. For instance, Vodafone's 200GB plan comes in at AU$39.75 p/m on average for the first year, meaning you get twice as much data as Optus' 100GB plan at a slightly cheaper price point.…