different prices for same product

We are selling a product where the first subscriber would be billed X amount, and each additional subscriber would be billed X - Y. In an ideal world, the customer would select, say 4 subscribers, and the cart would show one at X price, and three at (X-Y) price. Sort of like a "buy one, get one at half price".
Can this be done? If not, what is the best solution?