Attributes name[price]… how does it work?

Hi guys,

Anyone knows how to use the name[price] attribute to set different prices for different sizes p.e.?

I have tested and its changes the price but it keeps adding or subtracting if i choose other sizes.
And when i add it to the cart it goes with the original price to the cart in the top corner but if i finish the order it appears ok in the checkout page.

Something like this:
Product price 100€.
If i choose a sizes that has a +25€ it sets the price to 125€, but if i then choose a size that has a +50€ then it set the price to 175€ instead of 150€.

Thanks.