If your product has buyer-selectable options that affect the price, you can use our Variants feature to set that up:
You can always edit the product's settings in Seller Admin to alter the base price as needed, which would take effect immediately, and your existing button codes for that item should continue working fine. You should only ever need to recopy-paste fresh button codes if you change the product's item number, suggested price (if the item uses "Let buyer edit price"), or Variants/Variations configuration.
You can also set up a discount with the Discount Code field left blank, which would make that an automatic discount that doesn't require a coupon code; this discount would get applied to every Cart order that meets the Product, Min. Item Total, and Min. Item Qty. requirements you define for that discount, but it won't work with Buy Now buttons. This help page explains our discount settings in more detail:
Otherwise, if your page just needs to raise the item price arbitrarily without altering the product's base price setting in Seller Admin or setting up discounts or Variants, you can add an &amount= parameter to the button's href= URL, like so:
The amount you specify there will be ADDED to the base price for Add to Cart buttons, or will REPLACE the base price for Buy Now buttons. In the latter case, you cannot undercut the item's base price in Admin; while the buyer could proceed to checkout for a lower price, after payment our system would refuse to process the order if the price paid does not meet or exceed the item's base price in Seller Admin.