for basic handgun moulds, its about 1000 grams of silicone per side. That could be down to 800 or up and over 1200 depending on the handgun and your own mould making techniques. I do box moulds.
Then youll need at least a trial kit, or two pounds, of casting material. That sounds like a lot but in weight it adds up quickly. I just did some space pistols for a show and those were a little over 600 grams each. Again in material it depends on the item in question.
I use MKP materials. Others use Smooth On. I just tried a sample kit of Smooth On Smooth Cast 300 and it seemed to be Pour a Kast. Same smell, same viscosity, same color, same work time... Pour a Kast isnt bad stuff at all. The castings came out nice but shrinkage is somewhat shocking when demoulding. The shrinkage for both Smooth Cast 300 and Pour a Kast is visible in the mould on a cured part.
WIth MPK high impact plastic, that shrinkage isnt there. I use the stuff all the time on parts as little in size less than ten grams and up and over in pounds. You would want a slower setting casting material however so you can do a simple dump pour. MPK has a slower setting plastic. You can call him direct, info is on the website.
As for a kit, you would likely need two of these:
hobbysilicone.com