thanks for the plug
I came in here because I am about to start work on a Silurian myself
I am going to try and do it as per the originals so it'll be my first foray into two part silicon mixes XD
Honestly though $100 is very cheap for a DIY headcast! I did mine even cheaper by doing things I would not recommend! I am *not* going to go into how I did it for the same reason I don't talk about my first appliances (elf ears) because I did a dangerous thing in the process. But I also have a lot more work to do to fill in the eyes and nose areas and general cleaning etc. So the cheaper you go the more work you need to put in.
The absolute cheapest way to do it would be to make a plasticine sculpt on a wig form and directly lay latex over that. You can do this, I have done it with a test sculpt of lekku (my molds need fixing along with my head cast so I'm saving my allergy meds for one day of horrible resin work). You get a shiny surface and not much detail but you can then have a flexible piece you can back fill with foam pieces or fibrefill for support and paint just like a normal latex appliance.
If you stipple with thickened latex on a sponge you will get a rough but more neutral skin texture too. It's fairly close to how I made my current lekku as seen in my facebook page. The latex will dry the plasticine and make it powdery so it's fairly easy to pull straight off even with undercuts. Just be sure to get an even coverage.
I have some info here: http://neimhaille.tumblr.com/tagged/make-up