What's happiness got to do with +ing gems? I have never used +ing Gems to boost my happiness ever. They are a performance enhancement item. If I walk into my local Speed Shop and buy a Performance part for my car I expect my car to perform better not break or fail to change anything.
So you get absolutely no happiness from jewels succeeding, meaning that if a piece of gear you had went from +0 to +16, that doesn't increase your happiness in the least bit? Then why are you paying good money for the jewels, sir?
Paying for +ing gems is simple economics. You pay an amount=the probability of success x the amount of happiness you get from that success. In this case, you are paying dias (aka real money) for the probability of success of the jewel multiplied by the amount of happiness you would get if the jewel was in fact successful.
My point was that it shouldn't matter if you are paying real money for some virtual item, because that virtual item provides you some level of happiness, and that is why you are willing to pay real money for it. The only difference with +ing jewels is that the probability of success is not 100%, but you still get some level of happiness if it does succeed. The left side of the equation must be equal to the right, so because the probability of success is not 100%, the amount you pay will also go down.
In your example with the car part, let's say you pay $100 for it and it is guaranteed to succeed. If the speed shop tells you there is only a 50% chance that it will make your car perform better, then you will only be willing to pay $50 for it.
However, this is not taking into account that there is a chance that the car part will actually make you worse off (ie lower the level of the piece of gear). Taking this into account would lead to a more complex equation, but the concept is the same. The higher the probability of you being worse off, the lower you would pay for the car part.
My point is that if you want all jewels to be perfect (no failure), then the price would also go up SUBSTANTIALLY. You're not really better off either way. But if you are willing to pay a lot more to eliminate the risk (which most people are, see the economic concept of risk-aversion if interested), then by all means ask for perfect jewels to be put in the dia shop.