I don’t know if I can add much to this thread, but I’ve been told, from various sources, not to give away rights to an illustration. I’ve been given a template of a standard contract which is a five year licence only. I don’t know if anyone can elaborate on this but I’m assuming selling the rights completely would come at an absolute premium.
That’s the theory, but the reality is a lot more complicated. Editorial illustration is normally a rights only, but rights are calculated based on scope of use (digital vs print vs how many copies are distributed) rather than time, or sometimes on both time and scope. In publishing there’s a lot of different rights: foreign rights, merchandising, film and television, etc... and these are normally listed in a contract together with the terms specific to each. That said, those are premium contracts.
Many solid and remunerative jobs are only possible on a “work-for-hire” agreement, where you give up all rights - for example educational publishers almost exclusively work that way: so basically if you insist on a rights-only agreement you’re not an interesting partner for them.
I hear from my agent that many small trade publishers are starting to work the same way, and I’ve been offered those type of contracts for trade as well...