they do mention possibly having to verify the financial proof for extensions, although it was in the section about following spouse visas. More likely to have to do that at an extension in country than arriving at a border I suspect, they don't tend to check very much at the borders.
from what the MFA has been saying so far, everything points to that they accept people can leave and come back immediately. I was wondering myself on this point, whether they wouldn't want people doing this, but everything they have put out about it, or when they have been asked, they have said yes you can do border bounces with it and get another 180 days.
it's very possible the extensions will be 1,900. The announcement in the Royal Gazette stated the visa fee was 10,000 but made no provision for a different price for extensions, and there has been nothing official to suggest they will cost that. I suspect this was misreporting at the start. Siam Legal reckon 1,900:
the 500k (equivalent) you show in a foreign bank outside Thailand when applying for the visa (which can only be done outside Thailand). It doesn't have to be in a Thai bank.
technically there are no refunds on application fees but in practice most embassies will not accept an application if they do not think they are going to approve the visa.
One other thing though about extensions in the Gazette is it mentions having the 500k at extension time, but only in reference to a spouse or child, so maybe it's only if extending those categories.
"(2) Accompanying persons of the aliens under (1) who are legal spouses and children who are not more than twenty years of age
Aliens under (1) must have financial evidence. or evidence of financial support For travel or having a guarantor of not less than 500,000 baht according to the Ministry of Foreign Affairs. and the Immigration Bureau jointly determine When submitting an application for a visa or requesting an extension of time Permission to stay in the Kingdom temporarily"
I know there was talk of that but there is no mention of a 10k extension fee in the Royal Gazette announcement, only 10k for the visa, and that it will be extendable once per entry (no price). Every other extension is 1,900 so I'd suspect this one will be too and the original talk of 10k extensions was mistaken. We'll see soon enough I guess.