I'm on an SETV, which I have just extended by 30 days. I can book a flight home on a lower rate than other days, but it's 2 days past my visa expiration - that is, expiry on July 21, flight on July 23.

 

I know the following day (July 22) is usually accepted without penalty, but what about 2 days (July 23)?

You will be fined 1000 baht for a overstay of 2 days. If less than 24 hours there is no fine if leaving from either airport in Bangkok.

To avoid the overstay you could apply for an extension, pay the 1900 baht fee and be granted 7 days to leave the country after it is denied.

Personally, I would avoid the overstay. Some members often say its no big deal ....just pay the fine etc. I get that advice. 

However a short overstay here and there can sometimes bite you. For example if you ever wished to obtain an elite visa a few overstays all be short could be issue. The flight price difference would want to be worth it.