Global price in one product option

Hi,

Let's see if somebody can help me, I'd really appreciate it.

I need one option to increase the global price of one product, no matter if my client adds 1 or 100 units of the product, the option price must be always the same.

Example:

My client ask for 1€ product.
One option of this product increase 10€ the unit price.
So if he adds 1 unit it is 11€. But if he adds 10 units, opencart shows 110€. I dont want that option to be increased, the price must: (10 units x 1€) (+ 10€ option, no matter the units).

Any idea?

Thanks in advance.