I just can't see where $2.50 is worth anyone's time. Even if she doesn't have much going on, it's still something she has to plan around. I just wonder if the other neighbor who gave you that pricing has a kid doing it or maybe it's a close friend who wanted to do it as a favor and wouldn't accept anything more than that. ??
I have walked/watched my neighbor's dog for free because we are close friends and I was available. But it's not something I would do regularly for others unless I was being adequately compensated for my time.
I do not live in a high cost area and the cheapest I could find for someone to do this was $20 per visit. (I could have hired a teenager for $10, but I couldn't find any that were available during the school day.)