Price after custom field configuration is wrong...why?

[magoonebbia]

I read all posts published but I don'understand why the price is wrong. I create a custom field string for 2 option product:
50 gr. must be €5,00+10%tax and 100gr. must be10+10% tax where I mistake? If I add the 100 gr. to cart the price is
the same: € 5,00 instead € 10,00...I'm desperate. Please tell me how configure the product and his customfield (what kind?)
with weight variant and his price...
Thanks in advance...I'm desperate because there are weeks thata I have this problem...

[attachment cleanup by admin]