I love the swiffer wet jet for many reasons, but two of those reasons are not the cost of the solution refills and the cost of the pad refills. So I did some searching and this is what I have found to work
For the solution: once the bottle is empty, bore a hole in the top big enough for a funnel to fit, then add your own solution and then just stick a cork in it. If your cork is too big, just trim away on the cork or you can make your hole a little larger. But I recommend manipulating the cork versus the bottle. I personally like to use a white vinegar and water solution, but you can just use your favorite bottle cleanr and dilute to package directions.
For the pad: You can go several ways with this, but something reusable seems to be the cheapest. I use plain old bar mops for dishtowels, and after one gets too stained, it makes an excellent replacement pad, just clip the backside with clothespins, binder clips or whatever you have handy, and when you're done, toss it in the wash. You can also use old washcloths, microfiber cloths, or even old cloth diapers.
Happy Swiffering the cheap way!