While Jack could easily offered to buy the meal, he had more than enough money to do so, he knew that Miguel was not the type to take a free meal without having worked for it. So he did not offer, did not even really think to offer, it just was not the way that their relationship worked. If it had been at Jack's family's house, then sure, they would have all shared gladly. Eating out was just a different realm, with different social rules.
For the most part, Jack's mind kind of glossed over the 'costume' part of that statement. It would not even register until later that afternoon.
"/What time's this party?/"
He waved over at the waitress, who had actually left not long before. Shooting her a wide grin, as his stomach growled obnoxiously again.