。
It's on me.
Literal
I [subject-が] treat [emphasis-よ].
おごる ('to treat someone, foot the bill') is the everyday verb for picking up the tab. 僕がおごる as a verb feels slightly more decisive than the noun version 僕のおごりだ ('my treat'); both work for the same restaurant-offer context. The が emphasizes the speaker as the unique payer — 'I'm the one paying' — and the よ insists on the announcement so the listener doesn't try to split the bill.