If you didn't buy a Visa BEFORE arriving, you have NOT a visa - you have a visa exempt.
If that is the case, when you go out of Thailand the visa exempt is over. You can come back in another visa exempt (max. 2 visa exempt per year if using land borders, no limit if using airport). Visa exempts are for free.
If that is not the case, and you DID buy a Visa BEFORE coming to Thailand, then you can get a re-entry permit in the airport before going through inmigration (1000฿ if I'm right), or you can just leave the country and come back in a (free) visa exempt.
Visa = you have a BIG sticker with the Visa, AND a stamp with the date you enter and the date of permited stay. It should be 2 months.
Visa exempt = you ONLY have the stamp with the dates, and it's ONE month.
Anyway, remember to have the three proofs when you re-enter: onward flight, acommodation and 20K฿ IN CASH.
The METV allows you to enter multiple times until the date of validity. Each enter is good for 60 days. You can extend an enter by 30 days.
So, if you leave the country and enter again the last day of validity, they will stamp for 60 days, plus 30 days extension. As the validity is six months, 6 months + 60 days + 30 days = 9 months.
Maybe it's worth to say that you don't need US$, but if pay with THB the price is higher. And, they only accept crispy new notes, reject notes with tear, writes...
For the Thai consulate, it's mandatory to pay baht - it's easy to get baht in any exchange in Vientiane.
In fact, if you should fill a TM28 each time you "change your address" that implies EVERY FARANG should fill it EVERY TIME they stay in a hotel. That's absolutely undoable.