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?