A Google search for "cart abandonment rate" turned up statistics that anywhere from well over half to three quarters of online shopping carts are abandoned, so what you've described sounds pretty much par for the course. That's not to say those are necessarily "lost" buyers; they may just be "window shopping" and may return to your site later to make an actual purchase.
There's no good way of targeting buyers who abandon their order, because they don't take any specific action to indicate they want to abandon their order; they just close the window/tab or navigate away to another site. While you could add some javascript to your page to pop up a message whenever anyone tries to leave your site, most people find that sort of thing rather annoying, so it may well do more harm than good, and it still wouldn't catch the buyers who take their order to checkout but then abandon their order there before approving actual payment.
If you're using our standard cart button codes that display the cart as an overlay "inside" your own page, then the cart is just a part of that page, so adding code snippets to your page would be the same as adding it to the "cart page". It's not really feasible to add javascript that would do something within the cart overlay itself, as that overlay is already being generated by javascript, but you can at least add custom HTML to the cart:
http://www.e-junkie.com/ej/help.custom-cart.htm
