Mr. Wang ordered 80 pieces of a certain commodity priced at 100 yuan each from the store. Mr. Wang said to the store manager: "If you are willing to reduce the price, I will order 4 more pieces for every 1 yuan price reduction." The manager calculated , if the price is reduced by 5%, Mr. Wang’s profit will be 100 yuan more than the original one because he ordered more. May I ask: How much is the cost of this commodity?
王先生向商店订购每件定价100元的某种商品80件，王先生向商店经理说："如果你肯减价，每减价1元，我就多订4购件．"经理算了一下，若减价5%，则由于王先生多订购，获得的利润比原来多100元，请问：这种商品的成本是多少元？
<gadget id="calculator">5 * 100</gadget>
<output>500</output>

<gadget id="calculator">500 + 100</gadget>
<output>600</output>

<gadget id="calculator">100 - 80</gadget>
<output>20</output>

<gadget id="calculator">600 / 20</gadget>
<output>30</output>

<gadget id="calculator">100 - 30</gadget>
<output>70</output>

<result>70</result>