Hi,
You can't write PHP code in the formula.
Supposing that you have two values in your field: 0 (with the title "no") and 1 (with the title "yes"), and supposing that the column name of your custom field is "custom_field" and that your custom field is a custom field of the table "item", you could write this formula:
{price}+6.25*{custom_field}
If the value selected is "no", then the formula will be {price}+6.25*0 and thus the calculated price will be equal to the base price of the product.
If the value selected is "yes", then the formula will be {price}+6.25*1 and thus the calculated price will be equal to the base price of the product plus 6.25.