What are the rules for re-entry on a multiple entry tourist visa in Thailand, and what are the financial requirements?

Use a Thailand border run service if wanting to do same day. More reliable if you are up around your 6 month and trying to squeeze in the extra entry at the end.

I came to Thailand on a 6 month tourist visa, I done one run to Cambodia stopped 1 night in Cambodia when returning the Thai immigration commented that I had only stay for 1 night but still let me into Thailand.

After the run to Cambodia I got a retirement visa,much easier.

There is no time requirement to be out of Thailand. There have been reports of people getting stopped and denied entry if they have spent almost 6 months in Thailand and tried to leave and return one last time before the visa expires to get a new stamp though. Immigration doesn't feel they are tourists after 6 months in Thailand.

As far as money in the bank, that will be up the the embassy you are applying at. Also note you can only get a Multiple Entry tourist visa from your home country Thai embassy. Check their website for the requirements. Most embassies stopped requiring financial evidence for this visa, but you would need to check with yours to be sure.