Budget means shore diving, and that means BIG ISLAND!! Seriously, Maui is OK but not even in the shore diving league of the Big Island. You could fill several days with good Maui dives, but the average BI spot is better in my book.
Take all your own gear except tanks - it's $5+ per day to rent a weight belt so if you're going for more than a week, USPS Medium Flat Rate Box Priority Mail the weights for $13 insured each way (don't forget to reinforce the box a lot with good tape). If you can arrange a good rental package deal in advance, it might be worth saving the airline luggage fees if you can drop a bag, depending on how long you're staying. It doesn't hurt to call and ask.
You can get a nice 1 BD condo for much less than $100 per night (ours was $72 total/nt, on the water, immaculate grounds, nice place), which would be hard to beat even with a low-rent hotel, and you get a kitchen and it will sleep two or 10 guys better than a hotel room. Occupancy is down - negotiate.
Car rental varies a lot, I've seen $11 per day up to nearly $30, and can change daily so set up a notification with Hotwire or the like and start shopping early. I've had decent luck with Priceline name your price, but make sure you low ball them seriously, or no deal. I try them last, since you pay on the spot.
Oahu used to be a $100 cheaper flight than the other islands, but it seems like that isn't so hard and fast anymore. There are lots of direct flights now to Maui and the BI. Just after Thanksgiving this year, Alaska and Hawaiian were offering Kona RTs for well under $400 from San Jose and Oakland. Flight cost is all about which day you fly so watch the fare schedule calendars regularly, and make the ticket the first thing you buy. Sign up for some daily travel deal emails too, so you get tipped when fares drop. I can't say whether buying early or late is better. The last couple of years, it's worked out for me to buy at the last minute.
Final budget suggestion: post a Craigslist ad for your car, gear, even room and boat dives - you might get good offers from some locals.
