Tips are low at local massage shops in Thailand

One thing to keep in mind here is
that it's not about whether the shop is cheap or expensive,
but rather the location itself—whether it's a tourist area or a local area—that matters.
In tourist areas, the unwritten rule is a base tip of 50 Baht per hour
(and sometimes they even seem slightly dissatisfied with that amount -;)
In local areas, they are quite happy even with just 40 Baht per hour.
That's why I usually go to massage shops in local areas that charge 200 Baht per hour
and give 40 Baht per hour as a tip. Even so, the service is very good.
Anyway, don't overtip when you go to Thailand.
It will definitely have a negative impact.
Even if you received good service, just give a little.
(Even that "little" amount is considered a lot by Thai standards)
That way, it's better for everyone in the long run.