How to dynamically calculate price?

Hi there, I use WP together with WooCommerce with Gravity Forms and Product addons (for gravity forms) and need to solve this issue: I have product (new years card) what has basic price 9,90 and no matter on how many pieces you order. But there's also possibility to print there own text, but from 30pcs above and pricing goes like as more pieces, as cheaper card is.

So here's pricelist:
basic 9,90/card
custom text 30-49 pcs 15,50/card
custom text 50-100 pcs 12,50/card
etc.

I was searching solution and it seemed that Product addons will help me but not exactly. My problem is that gravity form cannot touch on number of pieces so I cannot do logical conditionals based on quantity.

I need let customer choose the card, open detail and select one of two radio buttons - I want a basic OR I want custom text. If custom text selected product count must change into 30 at least. Or, this option can appear only if 30 of quantity is selected. No matter. At the moment, I welcome every idea to it.

Check on: http://pf.razitka-vizitky-tisk.cz/obchod/novorocni-prani-dl-karta/dl04-motiv-stromecek/

Thank you Chris. So you think I should rather ask at woocommerce or to creator of plugin what works with woocommerce with your gravity forms? In fact, theese additional priced texts are just option above. I originally thought I could make it just we that Gravity Forms Products addons plugin.