Is a comma appropriate in "Sounds good, thanks."
Sounds good, thanks.
would only be used in informal contexts. It would not be appropriate in formal essay writing, for example, as Sounds good is a fragment. Thus, there aren't any rules per se.
Personally, I have observed a comma to be more commonly used. I also read commas as a short pause and periods as a longer stop, so Sounds good. Thanks. doesn't sound like I would say it. I know that is subjective, but as there is not a formal rule, I think it is appropriate.