Thailand offers accommodation in all price categories. You should always inspect the room (or better several rooms, sometimes the owners do not offer the best / cheapest rooms first) before agreeing on a price. In smaller establishments, also ask for the price agreed in writing to avoid problems when checking out.
The best prices (30-50% discount) for accommodation can be found in the low season in Thailand, which is between May and August, which also coincides with the monsoon season in the region, which is not surprising. The high season is between December and February.
Prices quoted are average prices for the country and vary by region and season. In small provincial towns there are no luxury hotels or resorts, while on popular island beaches it can be difficult to find anything cheaper than 300-400 baht, even in low season.
In rural areas, homestays are common. Generally, this means that you stay with your host or on their property in accommodation that is not commercial accommodation. As a rule, meals are included.
Bed and breakfast is usually the cheapest option, basic rooms cost between 100 and 200 baht per room per night (100 or less for a dorm bed). You get a room with a fan, an Italian-style toilet (often shared), a shower (shared or private) and not much else. The best guest rooms, especially in cities with lots of foreigners, have more amenities (European-style toilets, 24-hour hot shower, bigger room or even balcony, free Wi-Fi, sometimes TV, daily maid service, fridge), prices after that range from 200 to 500 baht. This makes them very similar to Thai hotels. The difference is that they are geared more towards a Western clientele and as such often offer varied (sometimes overpriced) tours, computers and/or internet access in the rooms or a restaurant on the ground floor.
If you are happy with the guesthouse of your choice and plan to stay there for more than a few days (especially in low season or in places where there are many accommodation options, such as Chiang Mai), ask for a discount; this may not be offered everywhere, but if it is, the weekly rate can be about 25% lower, and for monthly rates it is not uncommon to pay half. Normally you will have to pay for the whole period requested, but note that in Thailand it is not usual to pay in advance if something changes and you need to ask. So if early departure is possible (but unlikely enough to pay for a week or a month in advance), you should discuss this option with the owner/manager beforehand.
Hostels are not typical in Thailand. The reason is obvious: given the abundance of cheap accommodation and the fact that hostels are not familiar to Thais and therefore purely Western, the price of a private room in a hostel will be almost the same or even cheaper than a bed in a hostel dorm. You may get a slightly more westernised facility, closer to that of hotels, but at the price of privacy. If you insist on staying in a youth hostel, you can find them in major cities by consulting the internet. But don’t expect to find them just by walking down the street.
Thai hotels start at about 200 baht and go up to about 800 baht. The upper part of this range will be air-conditioned, the lower part will not. The main difference is that a hotel room must have its own bathroom, bed linen and towels are provided and there may be a hot shower. Most of the guests are Thai. TVs are available, except in the guest rooms, but internet access is less common than in the guest rooms and even less common free of charge or in the room.
Tourist hotels usually cost around 1,000 baht and offer the basic elements for a beach holiday: swimming pool, room service and colour TV.
Boutique hotels from 2,000 baht have proliferated in recent years, offering a limited number of rooms (10 or less) and more personalised service. Although these can be excellent, their quality varies considerably, so research is essential.
Business and luxury hotels, starting at 4,000 baht, offer every modern amenity imaginable and are little different from hotels in the rest of the world. Some, including the Oriental, Sukhothai and Bangkok Peninsula, are among the best hotels in the world. The most luxurious properties are also in this price category, with some of the best and most private adding a few more zeros to the price.