If you can get hold of Tomps products , they are the best balance between price and quality I have come across, they are a worthwhile product if your on a tight budget, your only looking at £10 GBP per 1kg kit of their budget silicone! http://www.tomps.com/value-budget-rtv-s ... -1079.html
You will find that postage will always be a factor when ordering casting supplies, since they are classed as hazardous materials for transport. Some delivery companies refuse to handle them. It is something you just have to live with.
hope the info helps