Aside from a used mask, which most people don't recommend (but is a serviceable choice), your only option is to get one of those cheap street hockey masks that Franklin makes. However, I know those are extremely unsafe for puck play and make the lower end "widow maker" masks look like fortresses for your head.
If you can't find a safe, used option...spend a little more and get the Bauer 1200 if you're really on a budget. It's about 120 on inlinewarehouse.com (free shipping and 10% off if you join their membership by buying a tshirt or hat for 10 bucks). I, too, am a beginner roller hockey goalie and despite the claims that those are unsafe, it's good enough for beginner roller although I know I'm still taking a slight chance of having my head ringing if I ever take a shot to the mask.