What can Plaintiff do when can't afford to pay Defendant's legal cost?

First off, you may be overestimating the amount of the costs.  Court costs generally don't include legal fees, but rather things like filing fees, so the amount probably won't be so large. If you can't afford to pay even this amount, however, your options would be the same as with any other debt (e.g. ignore the debt hoping the creditor won't pursue it, work out a payment plan or debt reduction with the creditor, file a bankruptcy seeking to have the debt discharged, etc.), except that if the Court  has ordered you to pay, and you do not, you could be held in contempt of court if the court determines that you really do have the ability to pay and are willfully disobeying the order.