Fly to Thailand in low season
As we all know flights are most of the times the cheapest when the destination country is not in high season, I've been to Thailand during rain season and I'm glad that I didn't get scared by the name of the season, because despite maybe 5 days rain, where only half an hour to one hour it is actually raining, we had great weather! And we stayed for 5 weeks, so having 5 days with half an hour rain a day doesn't sound to bad eh? ;-)
Which kind of accommodation is the cheapest?
Obviously this part always depends on how you like to traveller! If you're a solo travel I would most likely recommend you to just stay in hostels, since these might be the cheapest option. But if you're traveling with other people up to four, five persons then you will get away waaaay cheaper with guest houses, since you're sharing the price of a guest house and not like in a hostel pay per person for the price. If you're a big group you might be able to find a cheap AirBnB or otherwise just book two guest houses! Always make sure to get an A/C and not a fan in guesthouses since it can get pretty hot!
Compare transportation methods
If you have enough time mostly bus and train will be the cheapest! BUT you can also find super cheap deals on flights! We flew from Phuket to Chiang Mai for only 25€! I would recommend you to check on Rome2Rio (also available as an App) which kind of transportation is the cheapest, but also check on your own - just in case! Especially with buses I suggest not only looking for the cheapest but also for the safest, since I've read already couple times that some people sneak in the trunk of the bus and steal things out of the backpacks! I personally used Lomprayah for bus travels and Sangserm for ferries! Another good website is skyscanner or kayak (also available as an App) to compare flight prices. As of destination within the city Bangkok is one of the cheapest followed by Chiang Mai and other places on the main land. On the island it can get a bit more expensive, especially Koh Phangan where you will always have to pay 100 Baht for Song-taews no matter how far you travel and with how many persons!
Luckily eating out in Thailand is pretty cheap! I recommend food stalls, since they do make really tasty meals, but always make sure it looks clean to not get food poisoned. For breakfast you will mostly have to go in a restaurant, which are more expensive than food stalls. So I decided to buy some cereals at 7/11 along with some soy milk and just kept one of the plastic bowls that I received when I bought some pad thai! If you're not a friend of this breakfast idea I would suggest you to buy some fruit at the market or buy one of those fruit smoothies that are offered literally everywhere! Also if you want to buy some snacks, check if a Tesco supermarket is around you, they are much cheaper than 7/11 and have a wider selection!
Cheap destinations in Thailand
I experienced Chiang Mai and Bangkok as one of the cheapest destinations! On the islands you will always pay more, but if you really want to avoid high prices I suggest not going to Koh Samui since this island is pretty touristic and it's hard to find food stalls! Either way you'll always find a cheap place to dine sometimes it will just be more difficult!