Well, when I am doing the comparison, I am talking US$. They're getting paid in local currency, but they are getting bupkus compared to what you and I would make at the same job in our respective locales - no matter what currency.
Specifically, I'm talking things like Nike importing a simple factory (say, TL7) to a TL6 area (say, Bangladesh). It uses the lower labor costs added to the automation, and can now make shoes a lot cheaper than if it employed people in England or the US. This is because of the standard of living differential - and, that relates (somewhat) to the TL differential.
And, again, that's without the added obstacles to mobility of Traveller's "comms=travel" and "jump takes a week" paradigms.
Edit: Those TL5 gardeners are getting paid in the hi-TL currency, as they somehow managed to get to the TL8 world. It's the folks working in the factories that are still "down the road" from their less well-paid family and friends. end edit