For Wavemaster(Majestic) a voucher costs 290,000 rupiah. Using a foreign exchange rate of 9,700 to the SIN $ that is SIN$29.90. There is the added cost of a higher cab ride of 30,000rupiah times two back & forth. So 60,000=SIN$6.18.
Total cost of using a voucher is 29.90 + 6.18 = SIN$36.08 VS $48
Up to you to decide if saving $11.92 is worth taking the extra effort