"Ad Bonus" is short for "Additional Bonus," and refers to the additional money you receive from having the Treasure Map item equipped (and possibly future items that involve additional money per bust). It's unlocked by researching the Pirate Ghost.
Each one costs $35,000, and every additional upgrade increases your total yield-per-bust, per-equipped-map, by $100 (Research Tier 1 = +$100 bonus per-equipped-map, Tier 2 = +$200, Tier 3 = $300).
I upgraded to Tier 3, and purchased 8 of them so that all four of my busters could have 2 at once, yielding a $2400 total bonus for each bust!
Although it is *very* expensive to research it to Tier 3 and buy 8 of them (between the 3 research tiers, and actually purchasing the maps themselves, it's almost $300,000, I believe)... it more than pays for itself in the long run! (it's all profit after about 100 - 150 busts)
I hope this helps!
comment good answer!