How Do I Whore Myself Out?

So one of my work colleagues offered me a small side gig, but wants me to sign a contract for the job, which would specify how much I would be paid for the work.

Problem is, I don’t know what a reasonable amount would be. When he showed/told me what the task would entail, it doesn’t seem like it would take more than a weekend or two and I don’t want to overcharge. But at the same time, I don’t want to shortsell myself.

Could use some advice.

Nissan pimping out their IDx for your time