I've lived on Maui since 1991, and even for a local it's expensive. I thought I'd share some of the tips for saving money - most are West Maui based (near Lahaina), as that's the area I live in. First of all, shopping at markets. In Lahaina you have just a few full on supermarkets: Safeway, Foodland and Foodland Farms. Safeway (located at Lahaina Cannery Mall, near Starbucks) has the lowest prices and the largest selection, but you'll want to make sure you have a Safeway Club Membership (it's free). Both Foodland and Foodland Farms don't have cards. Foodland is located near the other Starbucks in the Old Lahaina Center, and is more expensive. The most expensive is Foodland Farms, but it will also have a better selection of fine foods. This is kind of like the difference between Von's and Pavillions, Foodland Farms is more like Pavillions. As you travel north from Lahaina, there is Times Market in Honokowai, No Club Card here, and more expensive than Safeway, but convenient if you're staying in Honokowai. Then further north in Napili is Napili Market, owned by Foodland, but I think even a little more expensive than Foodland (maybe because the gas it costs to truck stuff up there?). Again, no Club Card. So of all the markets on the west side, Safeway will save you the most money.
No onto eating out. Most restaurants will have happy hours to save on drinks, and lots will have early-bird specials. Eating out is generally pretty pricey in Maui, but there are a few tricks to save money. For example, eating at Hula Grill in Kaanapali is expensive, but you'll save money if you opt to eat in the Barefoot Bar. In Lahaina, a great place for sandwiches under $10 is Take Home Maui. It's a small local place that has the best cut pineapple in Maui, but they also make great wraps and sandwiches. If you're into vegetarian or health food, Choice in Lahaina is great but super expensive - you'll save money by going to Honokowai Farmers Market store (and they have a special deal on the hot food bar about 1/2 hour before they close it up). Plus if you type in Maui Deals and look for the org, rather than the com, site, you can get tons of 50% off and other dining coupons for Maui - just print them out to be sure they are accepted, some places won't take the digital coupon.
Accomodations. Personally I think VRBO is your best bet for deals, unless you want a full service hotel. Be prepared to pay in advance, and they tack on maid service and taxes, but even still, most times you'll get a condo with a kitchen (which saves some money for food on Maui). Sometimes Craigslist has a good deal on vacation rentals too.
Car rentals. This is hit and miss. I don't suggest off brand car rentals personally for two reasons: One, it's harder to get back to the airport if you're in a rush; Two, the cars are usually older and the savings aren't worth it. I would compare online, book with a company that doesn't require prepayment, and then when you get to the airport walk up to each booth asking what their cheapest rental car is. You'd be amazed at the difference sometimes. That's because at booking time, before your trip, they don't know what the availability is going to be. If it ends up being that there are tons of cars, you'll probably get a better deal walking up to the booths when you arrive. If there is very limited availability, then you'll be happy you have a reservation. This is my best advice to save on car rentals.
Shopping for gifts. If you're looking for super cheap gifts, head for either K-mart or Walmart near the Kahului Airport when you arrive or before you depart (give yourself extra time). Other options that may have more unique items but still be pretty expensive are ABC Stores, Hilo Hattie, and Whaler's General. If you're looking for something really special, made in Hawaii, I suggest Sand & Sea in Paia, on the way to Hana. They do have some lesser expensive jewelry for kids and stuff too, plus jams and jellies made on Maui.
Driving and gas. There is no way really to save on gas other than at Costco, if you're a member. Costco will definitely have the cheapest gas on Maui. It's located very close to Kahului Airport.
Activities. To save the most amount of money you'll have to go to a Timeshare presentation. Some of the magazines offer a $125 Dining Certificate if you go, and I'm pretty sure that you can also get deals on activities when you take advantage of that deal, as it's an activity broker that offers the deal. Other than that you can look for coupons from the Maui deal site I mentioned earlier, and some of the magazines like Maui Magazine have coupons too. Some companies like Captain Steve's rafting have last minute deals if you call the night before and they have availability, and most companies offer a discount if you book online. One word of caution - if you're looking to go to Old Lahaina Luau they are often booked weeks in advance, so don't wait until you get to Maui to make your reservations. And there are rarely deals on that show as it's one of the most popular on Maui.
And there you have it - my tips to save money on Maui.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.