Espero que gostem...
- Hey, i bought our tickets yerterday. They cost a small fortune...Anyway, Are you ready for tonight?
- ...I'm afraid to say but i can't go. I'm sorry to back out on you at the eleventh hour but something came up and i just can't get out of it.
- Ok......
- Sorry, i'll make it up to you. How about a dinner next saturday in the new restaurant?
- Ok, i'll take you up on your offer!
Vamos lá:
1) Cost a small fortune/Cost an arm and a leg = Custar muito caro!
2) Back out = cancelar algo/se recusar a fazer algo que já prometido
3) At the eleventh hour = de ultima hora, no ultimo minuto
4) Something came up = algo novo surgiu (muito usado para justificar algo que cancelamos/adiamos/deixamos de fazer
5) Get out of it = Se livrar de fazer algo
6) make it up to you = Fazer algo legal por alguem (geralmente usado quando desapontamos alguem em algum sentido)
7) How about = Que tal
8) take you up on your offer = Aceitar algo proposto
*Todas as expressões citadas são bastante usadas, nenhuma é rara ou incomum.

