The 520 is a decent device. If you feel you want something better than what's been given to you, you can either save up for a while to where you can afford something better, or you can get a job to earn more than your 35/mo allowance.
I guess we can start with what your current budget is: How much do you currently have saved up towards a new phone? Then decide what features you require that are not in the 520 you already have. From there, there is options such as used market, swap, and getting some money from selling your current phone.