Nicholas J. Michalak said:

My puzzlement isn't why Fett would take the job, but why would Jabba need a high priced bodyguard? Who is he so afraid of that he needs one? And if Fett isn't the most badass hunter in the galaxy, why would Jabba skimp on the protection factor if he did need a bodyguard? This is my confusion.

-NJM

Given that Jabba previously employed Gringo I don't think we should place to much faith in his HR department.

Fett showed up with Han in a Box (good Alice in Chains song) and cool armor and Jabba said "I like the cut of your jib. Be my goon on call!" and Boba decided that a room in the palace and a hot showgirl beat tracking down another bail jumper. And Jabba obviously has at least a small army of armed retainers already seeing to his protection, so one more badass fits right in.

Or maybe the bounty on Han's head, coupled with what he got from Vader gave Fett such a surplus of cash he bought shares in Jabbacorp and is now hanging out in the shareholder's suite.

I mean look at the array of weirdees and circus freaks Jabba keeps around him. He's like Howard Stern! Who knows why any of those guys are there?