To pick one you dont use in the US, "Its the dogs bollocks". The sentence is correct (assuming you know what bollocks are). Yet you still cant figure out the meaning from the words.
Could you give an example of a sentence using your phrase? Then it would be possible to comment on it. ...mine as well... and ...might as well... are both possible but would mean entirely different things!
He said "My dog is very clever." She said "Mine (is) as well."
He said "What do you think, should we jump over the fence?" She said (we)Might as well."
He said "My dog is very clever." She said "Mine (is), as well." Right. Use the comma. Otherwise, you're saying: "Mine (is) as well (as yours is)." And making a hash of it.
He said "What do you think, should we jump over the fence?" She said (we)Might as well."Also right.