Read Leviticus 27:8 Using Other Translations
But if he be poorer than thy estimation, then he shall present himself before the priest, and the priest shall value him; according to his ability that vowed shall the priest value him.
And if someone is too poor to pay the valuation, then he shall be made to stand before the priest, and the priest shall value him; the priest shall value him according to what the vower can afford.
If you desire to make such a vow but cannot afford to pay the required amount, take the person to the priest. He will determine the amount for you to pay based on what you can afford.