Hello,
I hesitate to give you a number, because it depends on a number of things. Let me just say that $125 is a very small number for someone who is travelling and being involved with rehearsals. I know accompanists who are paid that much for a couple of rehearsals and a concert, period. They are paying for your convienience as well as your skill.
You could either charge them based on number of rehearsals, the performance, and the fact that you are travelling with them, or, you can give them one rounded number that reflects all of these things. You don't want to charge too much if maybe the funding for the choir is somewhat small, but you certainly don't want to charge them too little that you don't get what the job is worth. Also, are they paying for your food? At a festival, eating may be expensive, and there goes your $125.
My suggestion is that you ask around to the accompanists in your area, namely the experienced people who can tell you what they have charged in the past. If that is not possible, I say charge in the highest range of your comfort zone.
I hope this helps, good luck!