Used GMAT books - New study materials for the GMAT can be a really big expense. List prices for review books are usually around $40, and can be more if you're looking for a certain name brand. At three study books that's $120. There's a used book store for the local library system near where I live that sells one year old, not written in GMAT books with CDs for $2. That's about 90% off the Amazon price and 95% off bookstore prices bringing your total down to a nice, manageable $6. Barring that you could look to friends, Craigslist or Amazon for gently used books. When I was done with mine I gave them away for free. You could be the lucky recipient of someone's books.
Pay once for the GMAT - The GMAT is a pricey test to take at $250. You don't want to have to pay for that twice. So study early and often and take plenty of practice tests to make sure you're ready for your big day. Study harder than you think you need to and get some sleep the night before so you only need to take the GMAT once.
Apply to as few schools as possible - The application fees (~$250), transcript fees from your undergraduate college (~$15), score reports for the GMAT (~$28), and, heck, even postage can really add up. Don't apply to schools just to see if you can get in and don't apply in general if you don't want to go. Applying to Harvard just so you can say you got in to Harvard is silly if you know you never want to leave the West Coast. Really research each school to make sure the money and all the time you spend applying is a worthwhile investment. I only applied to one school, this saved me a bundle of money.
Look into fee waivers - It can save you more than half of the cost of the two most expensive items on the list - application fees and taking the GMAT. Waivers are pretty uncommon but it can't hurt to try especially if you're straight out of undergrad or have a really low income.
Avoid extra costs - There are a lot of services, books, and other gimmicks out there aimed at business school applicants. Avoid them. If you feel the need to read advice about getting in to your top choice head to the internet or your local library instead of buying over priced books that are just one person's opinion. If you want help applying ask friends or family for advice or help instead of paying some consultant $100/hr to do it. Most of these business schools have been selecting students for a very long time and they've gotten pretty good at it. Check their websites for information (free and from the horse's mouth), take some deep breaths, and you'll be fine.
With some legwork and luck you can easily stay under $750 for the application process. Which is good because you'll probably need to pony up around $1,000 for a deposit once you get in.