Underwear can be expensive, but it doesn't have to be. Usually, one buys at least 5 pair at a time, and if you're paying $20-$30 a pair, it can add up to a new pair of shoes. However, there are ways to buy fantastically cute underwear for a fraction of that price.
The first place you'll want to stop at is Target. Target has a great assortment of comfortable, sexy, cute, underwear for as low as $4/pair. Plus, the more pairs you buy, the cheaper the price. They carry cute boy shorts, frilly bikini underwear, and super comfortable thong-cut underwear made of the softest cotton that never rides and never holds too tight. The line Gilligan and O'Malley are inexpensive and come in a variety of styles. Whether you like lace or print, whether you like plain colors or simple black bikini bottoms, check this line first. You can comfortably shop their online store, or best to go into your local Target and see the huge array they have in-store.
Also check your local department stores, like Nordstrom, Macy's or Bloomingdale's, and shop their sale rack in the underwear department. You can easily find a pair of $35 Cosabella underwear on sale for $8 (maybe not in the color you would seek, but it's a great way to buy expensive underwear on the cheap). Always popular Hanky Panky underwear can rarely be found for less than $25, but you can always find them on a sale rack when the new styles arrive. Like couture, underwear has seasons, and new styles replace older styles (and colors and prints).
Don't forget to always check eBay. If you have a brand of underwear that you like, you can often find jobbers selling "dead stock". The manufacturers have to clear out their stock and sell them to companies for pennies on the dollar, and these items are often on eBay. Have fun shopping!