Well, let's look at the difference between the two sentences.
I'm working overtime this week, and I've earned $200 extra thus far.
The week is presumably not over yet, and already the speaker has earned $200 extra. Now let's imagine expressing this about last week or just any week in the past. The entire week is already over, so using a word that means "up to this point" is odd.
Instead, what you want to say is that there was a point last week (or some week in the past), by which you had earned $200 extra. So it becomes something like:
I was working overtime last/that week, and I had already earned $200 extra by Thursday.
"By Thursday" is just an example, any expression about a specific time would do. It could be derived from context too
John came to the restaurant last Thursday. I was working overtime, and I'd already earned $200 extra by then (or by that point).
Note that "thus far" has other meanings besides "up to this point" - the preceding explanation has to deal with your specific use case.