Mutt was not suggesting you don't need to plan and take fuel for the alternate as you do, you do not however need to factor in the aircraft cost for going to the alternate as this will 99.9 times out of 100 not be a factor. Such as you wont factor in landing and handling at the alternate as a cost. As it is the operating cost for a single trip you want, the diversion will only be a cost if you divert, which is very unlikely. I have diverted once in over 4000hrs of flying and as it was a closure due to security reasons we held for a while also, something that happens a lot before the divert decision is made even if it is due to marginal weather, second approach before divert is also common. If it is marginal weather you would also be carrying extra fuel to allow for holding so you hopefully don't need to divert as things slow down when the weather is bad. A storm should not mean you have to divert on a short sector, the fuel penalty for carrying extra gas is very small so many operators going to where there are storms forecast would have at least 30mins extra before they got towards min divert fuel.