This simple Virtuemat VMcustom plugin allows you to add a custom price field to some products.

When added, if the product price is set to 0, the customer decides the price. If a price is set, the customer can add a value to the price. The input value utomaticaly updates the item price. Plugin supports 'on the fly' currency conversion and sends a warning alert if page page is left or refreshed with a price set without adding the product to the cart.