Are you 100% sure that santa claus is a fictional character? Do you have to be able to prove that father christmas isn't real before you can say you are certain that he doesn't exist. I know that there is not one shred of empirical evidence for the existence of santa, that is the reason I am sure he doesn't exist. If suddenly there was proof I will be happy to come back to this thread and admit I was wrong and change my mind regarding the existence of santa.
I would admit that the extremely broad definition that Essan would like to use for the concept of god would mean that comparison doesn't apply fully, since Essan would (at least if I understand his/her position correctly) like to include natural phenomena within his definition of a first cause. Using that definition then I couldn't be 100% certain. Using the definition of god (as most people would use the word) as a supernatural omnipotent force then I am as certain as I am with regards to santa.
Since science can only ever deal with empirical evidence then the supernatural will by definition (apparently Essan's definition of the word supernatural seems to differ from how most people woud use the word) always be something that science cannot assimilate, count or calculate probabilities for. For that reason santa, god and the tooth fairy all fall into the category of things for which there is no empirical evidence, I cannot assign a probability to any of them since there is no data to use. However that lack of data, is meaningful in itself.
Essan perhaps you could tell us whether you believe gravity to be a supernatural force? It sounds like you would like to define the supernatural as anything science doesn't understand.
A lack of evidence is as useful in science as a mountain of evidence, I used a lack of evidence to prove something last week. It simply does not take faith to disbelieve something for which no empirical evidence has been presented.
Apologies for a disjointed reply, I was attempting to cover a lot of bases.