New Type Of Prices

i need add to my shop new type of price. for example, if we have some get parameter in url (/products?aaa), we show user one price of the product(for example, 10$). and if we haven't, show another (20$). and exactly that type of price(10 or 20) is added to the cart, when user click on button 'add to cart'. how can i do this? how can i change the price's value that's added to cart by some condition?