This info is probably somewhere else on this site but I know I don't like being led around to try to find a thread that's weeks, if not months old so, I'll help as much as I can.
I am also just starting on my first sculpt so, I don't know how much latex or Ultracal you really need. But, for just a pred mask, 20 lbs of clay should be enough. Also, are you doing a full face mask, a half mask or just a biohelmet? I'm working on a full face mask and I'm cutting it pretty close with 20 lbs right now. If you're doing full face, I'd probably recommend to get a little more clay just because it's better to have a little too much, rather than not enough, especially if you have to ship it in. (And if you're doing a full suit or larger sculpted pieces later, you'll need to buy more clay when you do that anyway)
You could probably wait to hear from other more experienced members before making the final decision though.
I believe a gallon of latex should be enough for your normal-size mask. It might be more than enough, but I don't remember the exact amount, and that'll depend on your sculpture anyway. As far as Ultracal 30 goes, I think that 25 pounds should do it, but that also could vary. I eventually found a place locally that sells 50 pound bags of Ultracal 30 at a reasonable price, so that beats mail-ordering it and paying for shipping.
I sculpted up a P1 style head which is sitting on my bench untouched for months now, hahaha
but anyway. The entire sculpt I probably used close to about 50 pounds of Water BAsed Clay WED CLAY. Its cheap.
25.00 bucks for 50 pounds. It is easy to work with, you can rough out shapes very quickly. It dries out slowly so you need to occasionally spritz it with water and keep a thick plastic cover over it. The mandibles on the sculpt do not have any armature for them. I just sculpted them purely with clay.
The reason I used close to 50 pounds of clay for the sculpt is that I am trying to get it to be true 1:1 ratio with the original. Of course I am guessing but I wanted it big (to fit most bio helmets out there) Laguna Clay Distributors
See the link above for clay distributors in your area. You want to look for WED CLAY EM-217
The reason I use WED Clay is because in every picture of a Professional Sculpt I have seen Steve Wang using this clay with awesome results.
My last mold for a 1:1 Head I used approximately 30-40 pounds of Ultracal30. Gypsum Solutions Distributor Link
See the link above to find a local retailer for a bag of Ultracal.
Buy a 50 pound bag. They are cheap enough and you want to make sure: A. Have enough to make the mold and B. Enough in case of any mistakes or waste when mixing up batchs.
You need burlap to reinforce the Ultracal mold. It acts like rebar in concrete. Burlap is cheap, buy it locally.
Latex... You can get away with 1 gallon. You will have to dump the gallon into the mold and slush it around in all the cracks by hand with a brush or something. Rocking the mold and rolling it around making sure to get a good gel coating on the surface of the plaster.
Then you pour out the excess.