Eruchin - how they work
At the end of Bechukosai the torah specifies the halachos of eruchin. If a person says - ערך פלוני עלי - “I will give the value of someone ,” the Torah fixes the amount that he has to pay to the beis hamikdash. This varies according to the age and gender of the person whose value he is giving. Heseg yad If the donor cannot afford to give the full erech then the cohen estimates how much he can afford: וְאִם-מָךְ הוּא מֵעֶרְכֶּךָ וְהֶעֱמִידוֹ לִפְנֵי הַכֹּהֵן וְהֶעֱרִיךְ אֹתוֹ הַכֹּהֵן עַל-פִּי אֲשֶׁר תַּשִּׂיג יַד הַנֹּדֵר יַעֲרִיכֶנּוּ הַכֹּהֵן “If he is too poor to give the full erech then he shall stand the person whose value he is donating before the cohen and the cohen shall value him according to the amount that the donor can afford.” The Rambam in Hilchos Eruchin (3:6) talks about a case where a poor person obligated himself to give the same as a wealthy person: עשיר שאמר ערכי עלי או ערך פלוני ושמע העני ואמר מה שאמר זה עלי הרי העני חייב בערך עשיר שהוא ערך שלם ...