I just checked Kayak.com It varies period to period. I checked January, April, August and September. The rainy or monsoon season is from March/April to October. The lowest fares I found was in January 2007. The highest was August 2007. The fares tapered off in April and September. EVA Corp. consistently posted the lowest fare. If you have flown on EVA, it is part of the Evergreen group of Taiwanese companies. They have a merchant marine fleet, hotels, and now an airlines. The flight attendants are very pretty and pretty good about helping out during the flight. The other low fare airline is Philippines Airlines but the flight 26 hours total time. The others were China Airlines and United Airlines. I usually fly China Airlines myself. Nice service and good flight times. I have flown China Airlines 6-7 times.
Now, the reason for the lower fares is that the departure times are in the late morning to late afternoon and arrive in Bangkok from 10 pm to almost 2 am. If that works for you. If you want to arrive in Bangkok during the day. Then the flights depart from 10:30 pm to 1 am and arrive from 11 am to 3 pm Bangkok time. The price difference is USD 80 or more. For that much money you can save in not staying in a hotel for that night by leaving 6-10 hours later.
Delta airlines does fly to Bangkok but it is in cooperation with EVA airlines and China Airlines.
Now, that I have said that about hotels. That USD 80 difference is a 5 star hotel. Now it depends on where you want to stay. Try www.asiarooms.com and double check it against www.kayak.com I stay at The Emerald Hotel for USD 51 per night for a 4 star hotel. It's clean and away from the noise and traffic of downtown Bangkok. I am a 15 minute subway ride to Sukhumvit. It is close to many massage places. There is a 7-11 a 3 minute walk away and it has a good coffee shop.
Check the reviews for each hotel, the fewer negative reviews the better. What is good is that you tend to see what people look for from different parts of the world.
Check out www.bangkoktonight.com for hotels along the BTS Skytrain and MRT subway in the Maps section
Have a good trip.