A few questions (though I would ask them here as I'm sure other people will be interested):
1) Presumably you have an idea of how many maps vs. how many illustrations you want, what's this number? Curious as to where you pulled the 10% for maps vs. the 20% for illustrations (I'm quicker at doing an illustration, but a map can take me days from concept to final image), so presumably there's just a lot more illustrations.
2) The % royalty is based on sales per unit? If so what are you expecting the sale per unit to be? Are you aiming in the D&D supplements pricing (around $30+ per book) or DriveRPG publishing ($5+ mark). If you're aiming at selling them for $5/unit minus expenses (20%?) and as a map maker I get 10% of $3 then you'd have to be shifting a LOT of units for it to be worth putting pen to paper.
3) How many illustrators/map makers are you looking to take on? 10% for map making isn't that nice if it's then divided between you and another 2 map makers.
4) Are you a publisher or are you/your group/company doing the writing/editing/etc. that the other 70% of the royalties go to? If someone is taking a job on royalties it's nice to know whether the person you're giving your art to actually has any experience in publishing previously so that you actually make some money (more so if the price per unit as above is lower). Also, if you are a known publisher then simply having work published can often offset any lack of income gained.
5) You mention publishing stuff for "The Underdark", which is WOTC intellectual property, do you have an equivalent for your world/system or do are you paying for licensing to actually create and publish content with WOTCs IP (presumably the former otherwise you'll pretty much be making no profit if you're paying for WOTC licensing )?
6) What market/ online presence do you already have for your product? Similarly to above you mention a depreciating royalties payment each year, but if you have zero presence then it may take you a year to actually get your product noticed and the higher sales coming in.
7) What time frame are you aiming for to have each compendium completed? How much of the content/writing is already done?
Either way, good luck with your project.