1. You can't always make a frag "stronger" or last longer without changing the character. Just check out some of the popular frags that are release in various strengths; they almost always smell different depending on the strength.
2. I find nothing wrong with synthetic chemicals; virtually all fragrance houses use them extensively.
3. Yes, synthetic chemicals are most often cheaper than natural, but then again, perfumes cost far less to make than they do to package or market. Without synthetics, you would have a problem making any sandalwood or oud based frags since the raw materials are in short supply and very expensive. How many dollars of raw material do you think is used to make the typical $100 bottle of perfume ( most often I would guess it is certainly less than $10 and in most cases less than $5).