How much to charge for my music in a game engine?

[Lord]

This is my first post here, and I'd just like to say 'Hey!' to everybody.

I'm a composer working on music for a game that will be made with a new engine.
The engine is not yet completed, and I have been asked by the programmer if I would like to include my music in a separately-purchasable Asset Pack that will also contain textures, scripts and models.

The developer asked how much I would charge, and I thought it reasonable to charge in commission.
Alternatively, I could set a flat fee for the songs and include royalties. At this moment the budget for the engine is fairly low.
The developer has also asked to have the exclusive rights to my music.

Just wondering if anybody has experienced something like this before and how you would go about payment.

Recap:
* The music will either be sold in a pack containing other non-musical items
* It's being sold separately from the engine
* The developer will get exclusive rights to my music