You can check out Evertek. They specialize in wholesale computer parts. There's a website called CPU4U.com that sells wholesale computer parts. They should have what you need.
Evertek provides wholesale MP3 players, both generic and off-brand. You can visit their website to learn more about your wholesale options, the link is http://www.evertek.com/products.asp?cat=MP3.
The cost to actually manufacturer to mouse totally depends on what kind it is. Wired or wireless, number of buttons, any special features, bluetooth, swiping, and included accessories all contribute to the cost. All mice are optical now. Ball mouse have not been made in several years. A 2 button wired mouse is by far the cheapest to make. Companies like Logitech and Microsoft design mice and have thousands or millions of the same model made, which reduces costs, otherwise, the design, manufacture, marketing, and shipping, costs would be very high per unit. Just the actual parts needed to make the most basic mouse, not including all of the things I mentioned above, usually costs less than a dollar. Plastic is cheap, and all those components inside, the LED, circuits, everything inside is mass produced and dirt cheap. The manufacturers are buying in extremely large quantity, which drives prices down. There is a company called Evertek that you can join if you are a business (they have a minimum order), and their prices are online. Their most basic mouse is less than $3, and that includes the manufacturer and Evertek's profit, so I guarantee you that mouse cost less than $1 to make. As with any computer hardware, you really aren't so much paying for the product itself, you are paying for the engineers time who designed it. The electrical and software engineers have high salaries and that's where most of the cost of the product comes from. This is why that $1 mouse goes for $10.