Patrick... look through the articles and posts in the SCRATCHBUILDING forum.... tons of info on getting into resin casting there. As a quickie, it will cost you about $50-60 US to get started... be aware that it is very EXPENSIVE to do this unless you are going to use a LOT of pieces from any mold you make - such as floors, tile roofing, etc... these are things you can use lots of over the years. But if all you want is an extra pillar, or to duplicate a model part, it is far cheaper just to buy another piece already made. On the other hand, it is a ton of fun to cast your own items! Here is my "bare minimum" list...
1) 2 part resin - i recommend Alumilite ($29 for one quart of each)
2) RTV Silicon - recommendation here is BARE METAL foils TinSil 70/20 - doesn't require a release agent when pouring plaster or resin. $40 bucks for 2 lbs
3) Some trays to hold your moldings