Optional price appears right but in cart it appears as basic price

I use woocommerce plus GF product addons and I have basic product price 9,90 CZK. I've done some conditionals I can, from 30 pcs do some extras for just some additional payment.

What means sine 30 pcs I use
basic price 9,90 + 5,60 in range of 30-49 pcs
basic price 9,90 + 2,60 in range of 50-99 pcs
etc.

and it works perfect. I see in cart I have 30 pcs with additional option for one price per piece in amount of 15,50 CZK.

So I thought it will work same if I reach such amount what enables you a discount. Like from 300 pcs above I count discount like

basic price 9,90 -0,90 CZK for 300-499 pcs etc.

With this, I see right count before adding into cart, BUT in cart I see just basic price 9,90.

Does anyone know what I do wrong?

Please check it here: http://pf.razitka-vizitky-tisk.cz/obchod/novorocni-prani-dl-karta/dl41-motiv-praha-karluv-most-hradcany/

Select radio button: s dotiskem vlastního textu
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No need to upload file, it is optional for logotype
and choose 300 pcs and put it into cart. Price should be 2700 CZK but is 2970 CZK what means it really counts with 9,90 instead of 9,00 CZK

Thanx Chris, I did too. It seems I need to use 0 price for product and do all counts via gravity forms. With this you may could help me with my idea to create a custom field for each product where I will put the price (as regular price field must be zero but products have different prices and I want to use just one form per category). I thougt I let gravity form populate dynamically via this custom field. Is this idea true? How would it look like than? I've found this snippet into functions.php?

add_filter('gform_field_value_your_parameter', 'my_custom_population_function');
function my_custom_population_function($value){
    return 'boom!';
}

Could you help me to have custom field eg. my_price ? How this snippet should look like?