Custom field price ALWAYS added to base price

[dafvm]

I have products in different sizes. Each size has a different price.
The different sizes and prices are setup via custom fields on the product.
I filled the total price of the product in the custom field value (thus not the incremental value +10 e.g.).
I have also setup the base price for the product in the product details, because this is mandatory.

Product A Base price is 100 euro
"Size 70x100 is 100 euro"
"Size 80x100 is 110 euro"
"Size 90x100 is 120 euro"
Now when I put 1 product A with size 80x100 in the cart the total price is incorrectly set to 210 euro (110 euro base plus 110 for size 80x100).

The solution would be to exclude the base price from the total price. How to do this?

The other solution would be to only use +10 euro, +20 euro etc. This would mean an unaffordable amount of work for me, so I hope solution one is possible.