There are a few variations on this, so let me talk about some of the limitations. I don't have a good grasp on the exact numbers, but hopefully this will give you an idea. I'm going to start with things that would allow you to re-use a mission, and what are some of the factors.
The first step is to have a rocket in assembly that can have it's mission changed. This would allow you to do a turn around a rocket as quickly as you make them, and doesn't add a lot of up front cost. It would cost quite a bit if you excersized the option, but if you need it, then it's available.
Next would be to have a rocket on the launch site. This would require a location to store the rocket. They are usually stored in a clean room of sorts. The expense is in building a second room, basically it would double the operating costs of the facilities at the launch location. This could still prove useful as it would allow you to do almost twice as many launches, processing two rockets at the same time. If you have one waiting in continual readiness, it reduces this effectiveness, but would allow you to launch a rocket even sooner.
Beyond this, you could have a mission on the pad, but not fueled. This would probably allow for a launch within a few hours. This could not be kept up for a long time, as the rocket would rust eventually, or have to be taken in due to weather constraints. If one of these two items happened, it would add considerably to the cost.
Finally, you could have a fully fueled rocket on the launch pad, ready to go. This would cost quite a bit extra. For the Space Shuttle, a scrub costs $1.2 million. Other vehicles are less, but it is still expensive. Thermal cycles add stress to the equipment, eventually leading to the potentials of failures. Also, keep in mind that this assumes a launch over a short period of time. If one were to leave the rocket exposed ready to launch for a longer period of time, the cost would go up.
Bottom line is, it all just depends how much you want to pay. I suspect that the readiness for the scenario you described wouldn't require anything more than having a backup rocket at the launch site. This allows for fairly quick turnaround of a launch, but not that quick. It doubles the cost at the pad, and might have some more subtle effects in increased maintenance for the rocket, but overall wouldn't break the bank.