Im doing a little project and im trying to keep my costs down as much as possible... (im poor) 

Silicon is a lot more expensive than I expected, I'm currently looking at having to buy 2KG  (2 x 1KG) of the Tin condensation cured silicon. But I'm not entirely sure how far that jar is going to go... How much volume is 1kg?

You see on youtube other prop makers and what not mixing up large buckets, several litres of the stuff, like its nothing... and then fail a mold and just carry on again like its nothing... Which gives the impression they didnt just waste 500 quids worth of silicon... Makes you wonder if the stuff just is that much more expensive this side of the pond... 

Anyway I was thinking. I could use pure silicon to capture the details of my model... then quickly mix in some kind of bulking powder to fill up the molding block. That way saving on the silicon. Sooo umm are there any cheap agents that i can mix in to do that?

What some people do is use a small amount of silicone over the part, then back it up with cheap fibreglass or similar as a jacket.

Generally it is easier to do that with addition cure silicones as you can get thickening agents to make it thick enough to brush on without running. You can then build up a couple of layers then make a jacket.

Most silicones have a density close to 1 so generally 1kg of silicone is approximately 1L by volume.  You can work it out exactly if you pull the density from the specifications.

In terms of cost, they may be buying in bulk which brings the unit cost down.  Also they will factor into the price they quote for a project a proportion to cover typical material failures etc.  something which as a DIY user you can't easily factor out. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant