The mask is without a doubt the most important part of any set up. If you are going to invest in only one piece of gear, let it be the mask. The quality of mask can make or break how you enjoyed the day. Cheaper masks tend to not be as comfortable, and even fog up preventing you from seeing on the field. As masks can not be removed for any reason while on the field, it is best to get one with a 'thermal' lens that will prevent fogging. There are less injuries in paintball- per capita- than there is golf and that is all because the mask, and it staying on your face, you want something that you won't be tempting to remove.
If you are looking to get something decent, we recommend starting at $35, as most at that price point and above have thermal lenses. Anything cheaper, and it will likely have an 'anti-fog' single lens that will fog.