Instead of the original sentence -
You better thank me, coz' if it wasn't for me, you would have been dead.
This sounds just a little better -
You better thank me, coz' if it weren't for me, you'd be dead.
Since being dead can be used in the present and doesn't need a more complex tense like "have been" in order to make sense.
If there is a preference to say "would have been", then you could say "would have been killed", since "killed" is an action which took place in the past, rather than the present.
Next, to choose between
- you'd be dead.
- you'd be dead by now.
- you'd be dead now.
Options 1 and 3 seem more appropriate for the sudden car accident scenario.
If there were a situation which took place over a long period of time, weeks, months, years, etc, then "by now" could be more relevant to explaining it. "Without that medication you'd be dead by now."