Realistically, in Bangkok a minimum of 2k English pounds. Outside of Bkk, 1.5k English. Of course, it can be done slightly cheaper, especially up in Isan, but why come here to live a restricted and limited life......
As everyone has posted above, don't worry. You've got more chance of being winning the lottery than being asked to prove funds. Of course, have something available just incase, or a lump sum of cash on you, but in reality you'll be fine. I've never been asked in 15 years of going back and forth.
I would just book a cheap Nok Air, Smile or Air Asia flight out of Bkk for around the 30 day Mark but not use it. At least you can show an outward flight if required. You can get a cheap flight with any of the above for about £50.