I'd made clear in a message that I can't afford upfront payment and would be interested in an ongoing royalty-split.
That's the mark of a real professional IMHO.
Audition anyway and state your fee. Maybe (probably) you won't get the assignment - but you've at least made the initial contact, demonstrated what a professional can do for a project, and let the potential future customer
know what your services go for.
Nothing ventured - nothing gained, right?
I'm a musician so I can appreciate where Aaron's coming from - even if I probably wouldn't have done the same pitch since you were quite clear there was zero cash available up front, and
you're a first-time author (so it's probably true.)