Industry standard is to pay for a "sample piece" as you called it (the actual term is an art test) unless it's like 99% sure that you're going to hire that person, it's just "final confirmation".
Depending on how many panels there are per chapter, 150$ is either a low or an insanely low price. For reference, industry standard prices for freelancers per lined page (no colour) range from 120-450USD, that's PER PAGE usually averaging 6-7 panels, not per chapter, and includes chara design, storyboarding and linework, but not lettering nor writing. I'm assuming you also want them to make character designs based on your character descriptions, which should also be paid.
Making art is time consuming and therefore if you want someone else to make it for you it's expensive. The person you're talking to is offering to work for dirt cheap. If you want 12 chapters done in a timely manner with pro/semi-pro level quality, you need a budget. If you want someone to do months of work for free, then you'll need someone who either needs the practice (less quality and usually less professionnal/efficient) or find a pro who loves your story so much that they'll do it anyway (and for that you need an excellent pitch and to find artists that don't already have a story they're working on).
