Boon of the Hoarder gives a chance to drop gold after killing a monster. Is this gold affected by Gold Find bonus?
Unless there is some outrageous special case, yes, it should be affected.
Gold drops are multiplied by two factors - character's Gold Find stat (from items and Paragon points) and difficulty. The reason I'm assuming that it affects all gold drops is that the value of Gold Find in character details screen is shown pre-multiplied by difficulty (try creating a game on different difficulties and check).
That said, it is not worth it to artificially invest in Gold Find on all items - just grab a Goldskin or another exceptionally "greedy" Legendary if it fits, or when going into the Vault. You are usually better off rerolling primary stats, so it does not really matter if you have Gold Find on an item or not - you won't be able to change that, if you reroll a primary.
Raising the difficulty, meanwhile, gives you more things than just more gold.