If they said they were going to take you to get a pedicure, they committed themselves to it, and when they went without you, they broke their promise. However, if the plan was about going together, and nothing was said about paying for you as a birthday present or definite arrangements made, then perhaps you had a different understanding than they did. From the sound of their reply, it does not seem that they had planned to treat you, and it sounds like they felt the cost was more than they chose to spend. And it also sounds like they did not feel that the plans were firm.
I would be hurt to be excluded from what I thought was a planned activity, but I can't say the idea of a pedicure has a strong appeal. More of a mixed one--I worry about fungal infections which are a bear to get rid of--if you ever can.
So, find another friend to invite to go pedicuring with you, and treat yourself.