The how-to section on the the hunters lair has numerous threads about methods of making a mask. There are threads that have Pep files of various bio helmets - which some come up very accurate. In regards to the undermask - I would buy a half mask.... save yourself the time and effort plus as sleepalot says they are cheaper in comparison to the full head masks with full facials, teeth and mandibles. Plus, sometimes, if you there at the right time, you can land a deal that gets you a half mask, bio, dreads and neck ring.
But as stated above, the hobby is expensive by any stretch - even on a budget. I have been building a predator costume for about 2 1/2 years and I started on a budget, buying bits here and there and I am still going. The only issue I have come across, is continuously changing bits to make them look better. I have now taken to sculpting - mouding - casting various bits including masks.