Can I leave Thailand to Singapore and return the same day on a 60-day multiple entry tourist visa?

You'd be fine if you went out the same day and came back on the same day even much earlier, there is no rule you have to be out overnight. If you did it at a land border you could spend as little as 5 minutes out of the country.

Fair enough, that's a good price then for sure!

Yes if you plan to leave after your next 60 days better to bounce than extend. Do note as well though that you can just do a land border bounce at either Cambodia or Myanmar, you don't NEED to fly to Singapore. If you are doing a round-trip come straight back don't leave the airport sort of trip Kuala Lumpur or Yangon is usually the cheapest, quite a bit cheaper than Singapore. A lot of other options as well- Penang, Ho Chi Minh, Vientiane, Hanoi. Yangon you can get $70 return, most of these others are under $100. Singapore is usually more like $200.

You have a multiple entry visa. You can enter for 60, extend for 30, then leave and come back for 60 and extend that for 30 again. If you re-enter before the 'use by date', you can get more time from your visa.

You can extend only once per entry stamp, but it's the entry stamp, not the visa. You can extend each entry stamp you get on it once, it's not limited to once per visa. So you can do 90 days per stamp, rather than 60.