You’ve got a few options.
1 - You cancel the order. If you really can’t afford to complete the order then this is your safest option. However cancellation isn’t recommended as it can have a negative impact upon your gigs and seller level.
2- You can ask the buyer to change the price. Whilst this seems like the best option in principle, the buyer is unlikely to do this and they are entitled to decline (which I suspect they will).
3 - You do the job well; you get a good review.
I recommend just doing 3. We all make mistakes and next time you’ll be able to make a profit on your order by learning from this.