Hello,
For values in drop-down custom fields, please prefer a less complexe content, such as "0", "1", "2", etc.
It will avoid some kind of misspelling issues.
Otherwise, we made a simpler test with a product used by the field you gave us (since we cannot check the elements for the product you gave in your first message).
Here our sample :
pasteboard.co/TN8CFObGFoJ6.png
Please note that you have a tax category set-up, which can influence the final price.
And since it looks like you're using the "taxpay" plugin, I do have the feeling that's a tax issue and you should give more details regarding that.
You can also configure the "custom field price" elements to not have a tax category and see if the price is better.
Regards,