In Greetings^x

Hi, i'd write "Atentamente" or "Un cordial saludo". Both are formal, the latter is perhaps more "cordial". Warm greetings,

for a informal greeting hola tio ¡ ¿como te va colega? for a formal greeting buenos dias

I just want to know a couple of things when greeting people. For example let's say it goes like this: Como estas? Muy bien gracias, y tu? el mismo Is the use of "el mismo" a common response? Or the use "el mismo" in response when some says to have a nice...