Why does Ishaan need Tanvi to come?

Ishaan: So it's settled. Manali, the second week of June. All five of us.

Tanvi: Sounds lovely.

Ishaan: You'll come, won't you? We need five to share the car.

Tanvi: Of course. Well, probably. It depends on a few things.

Ishaan: Like what?

Tanvi: My cousin might be visiting that week. And the coaching classes might start early. And you know how my mother is about mountains.

Ishaan: Your cousin visits every June. You've never let her stop you before.

Tanvi: This time she might stay longer.

Ishaan: Tanvi. Is it the money?

Tanvi: (after a long silence) It's a bit more than I thought.

Ishaan: Why didn't you just say so?

Tanvi: Because it sounds so small. Everyone else said yes straight away.

Ishaan: It's not small. We'll take the bus instead of the car, and stay in the hostel, not the hotel. It'll cost half as much.

Tanvi: You'd do that?

Ishaan: The hills look exactly the same from a bus.