If the poster gets a prize, who gets it, the person presenting it or the first author?
A prize is typically given for both content and presentation of a work, so a typical fair way to handle prize money, whether for poster or paper, is to offer to split it evenly amongst all of the co-authors.  Co-authors who feel they have not offered a full share of work might choose to decline their share, but the basic assumption should be an even split.  To do otherwise just opens the opportunity for much interpersonal conflict over a generally quite small amount of money.

Remember also that the most significant value of a poster or paper prize is not typically any money that happens to be associated with the prize, but rather the increase in your personal credibility and future prospects that comes from the recognition.