Assume that the 15 percent consumption tax will be tacked onto your hotel bill. An additional 2.5 percent room tax covers the salaries of the staff at the Tonga Visitors Bureau. These taxes often won't be included in the amount you're quoted when you ask the price of a room and will be added when the time arrives to pay.
On this website we've organized the accommodations listings by price, from low budget (under US$25 for a double room) to budget (US$25-50), moderate (US$50-100), medium (US$100-150), upscale (US$150-250), and deluxe (over US$250).
Although Tonga is a bit more expensive than Samoa, there are some beautifully situated places to stay. Prices at Vava'u are generally higher than on Tongatapu, 'Eua, and Lifuka, especially during the whale-watching season. Unless you're prepared to share a dorm with other backpackers, advance room reservations are a good idea from June to October, especially at Vava'u. Accommodation bookings made by airlines or local travel agencies often don't go through and direct contact by email or telephone works better.
A few of the budget beach resorts will allow you to pitch your own tent on their grounds. Freelance camping is prohibited. Unlike in Fiji and Samoa (but as in the Cook Islands), you'll rarely be invited to spend the night in a local home. It's not forbidden to stay with the locals, it's just that the Tongans prefer to keep a certain distance between themselves and palangi tourists. If you do stay with a family, you should be aware of theft. On the plus side, you won't be expected to conform to a lot of complex social mores like you will in Samoa, and people tend to leave you alone.
Several of the guesthouses in Nuku'alofa have cooking facilities. At Ha'apai and 'Eua, most accommodations do allow cooking, but grudgingly, and they levy a small per person charge for gas and electricity. Several places at Vava'u allow you to cook. Neiafu and Nuku'alofa have good public markets.