What is a word for someone that only does something to benefit them?

Solution 1:

Self-serving - MW

Having or showing concern only about your own needs and interests.

There are a TON of synonyms for selfish, pick yours: Thesaurus selfish. In my opinion, self-serving fits quite perfectly into your description.

Solution 2:

Mercenary — ODO

noun 1.1 A person primarily motivated by personal gain
"cricket’s most infamous mercenary"

Solution 3:

In the case of your example paragraph:

I am a follower, not a leader. Hell, I'm not even that good of a follower either, just someone who will do your bidding for the right price. Even then, if it doesn't benefit me -- or put you in debt to me -- I won't do it.

Moneygrabber

Fits well, which is a countable noun derived from:

Moneygrabbing

Very interested in money and trying to get as much money as possible. - Macmillan