WooCommcerce + WordPress Custom Product Attribute Validation

We have a WordPress + WooCommerce setup running the WooCommerce Subscription addon. We have a variable subscription that has several attributes. We want to customize the product page and attribute dropdown to include custom validation before allowing the user to add the product/subscription to their cart. For example, the user can select a total of 30 widgets (in quantities of 10) and once the variations are selected to add up to 30 widgets total, then the add to cart/subscribe button should appear. Before the quantity is met, the button should be grayed out and instead display "Add X more widgets". See 2 attached screenshots for how the user would select the quantity of widgets as well as the various states. Please let us know your estimate for both time & cost. Previous experience with WooCommerce, specifically subscription functionality, is a plus! Send any examples of relevant WordPress + WooCommerce work.