The Queen Needs an Unerak

A group of friends were trying to plan a joint 30th birthday celebration. One member started rejecting every option, saying that they were too expensive. She wanted to go earlier in the year so it would be cheaper - but her partner and i already have plans. What is the etiquette in these situations? Should we politely leave the group and let them go for a cheaper price? Or should we say to this member that we agreed a more expensive holiday)? Let me know your thoughts or the rest.