The value of SGD is not the main factor, it's more about the prices of (Goods &) Services.
I haven't been in the sg scene (I'm overseas) for 20 years already. Assuming 1 can get hj for sgd 50 previously. Now price of hj go up beyond 100 sgd, but yr salary / income remain the same, or not increasing at the same pace.
Many people would then of course look / search for options & alternatives that the hj cost much lower than that quote of sgd 100 or higher! Eg, going across the causeway, u spend much lesser than sgd 100, but get more (may even get fj)! Although exchange rate will decide how much yr sgd 100 can change into myr, but the fluctuation / difference doesn't really affect yr "purchasing power" to get that hj service. Eg again, whether exchange rate is 3.20 or 3.30, sgd 100 changes to 320 myr or 330 myr. Does the 10 myr make the difference whether u can "purchase" the hj service? sgd 100 not enough to get u hj service in sg, but u still have remaining if u go across the causeway.
The main factor is the living expenses in sg. The shop has expenses (rental, electricity, water, etc costs). The wl need to charge the fees not only to cover the costs, but also her income (cover costs only, work for free? costs not covered, "business" lost, sell her body not enough to pay the bills?). The wl's income, she also need to think of her living expenses.
The expenses in neighboring countries, ie, rentals, electricity, water, wl's income & living expenses, etc, compared with sg, need to say more?
With lower expenses, of course they can charge lower rates for the services!
The other factor is the wl's attitude. Some may take customers for granted / stupid, just want more $$. While some charge "market rates" or willing to be negotiable.