Based on the latest market prices tracked on TCG Stacked, Mockingbird is worth about $7.16 USD for a regular copy and $15.63 USD for a foil copy
Ive found that sticking to cards that just get the opponent dead, like Hero of Bladehold, and removal like Swords to Plowshares and Solitude to disrupt the opponent is more than good enough against most foes
Aveline de Grandpre is weirdly a Golgari card, I am only now noticing
A two mana lord which buffs all your creatures is a pretty standard rate, but Lord of Atlantis does much for you than that: he also gives everything islandwalk
Three mana dorks get you mana positive in one turn cycle, your creatures get virtual vigilance, and lots of other activated abilities get additional uses
Its also a strong beater to boot, something my own main Karador deck sometimes struggles with