Well that's a heck of a necro bump. ;p
All boosts are calculated as their own thing, then added. Merchant Prince gains "[Magic + 1] Gold, boosted by Red Gems destroyed. [10x]". So it's:
(Magic+1) + (# Red Gems destroyed * 10)
If the base amount is 2 gold (1 Magic +1), and you destroy 3 red gems (3*10), you'd gain 32 gold.
It's the same behaviour for anything that's "boosted", too. So if an Astral Spirit deals "[Magic + 2] damage, boosted by gems removed [3:1]", then that's (Magic+2) + (# Gems Removed / 3), etc. etc.