• 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

You could sell them the music but then ask for royalties beyond a certain amount. That way, the developer can stay within their production budget and if the game is a flop then you wouldn't have made much in commission anyway but if it's a success then you would get a share of the profits. Also, the fact that they want a "separately-purchasable Asset Pack" ...or exclusive rights to your music... means that if they don't end up using your music then they have the right to liquidate it and sell it to someone else to use and so on and so on and the next thing you know it ends up in a porn film. Unfortunately up-and-comers (not suggesting that you are) have to sometimes take the best deal we can get even if it means selling a package (not without negotiating a good price first) and then just focusing on the next project.

Good luck to ya!

Hey,

I just read this previous thread:

http://www.gamedev.net/topic/678393-game-music-and-sound-survey/

And on the survey website you can see their results of their 2015 survey here:

http://www.gamesoundcon.com/#!game-music-jobs-salary-2015/zksrq

It provides a lot of good information about how music and sound design jobs looked in 2015. It's worth looking over to get an idea of what kind of money you can make and what types of contracts were typically made to license the music.

Hope that helps.