It's on me.我来出钱。/我请客。
A: Order what you want. It's on me.
想吃什么尽管点，我请客。
B: That's very nice. I guess I'll get the steak.
太好了，我想吃牛排。

How much do I owe you?(餐馆、商店等）多少钱?
A: You never paid me the money I lent you.
欠我的钱，你一分都没还呢。
B: I'm sorry. How much do I owe you?
不好意思，欠你多少来着？

(This is) My treat.我来请。
A: How much is the restaurant?
这顿饭多少钱？
B: Don't worry about it This.
别担心，我请你。