Upvote:4
FX spreads will be tight in venues where the currency pair has liquidity, and will be wide where the pair is less liquid. As a general rule, currencies are most liquid in their home country and if you are holding MYR and want THB, you will can expect narrow spreads in Malaysia or Thailand, and wider spreads as you move farther away from those venues.
The MYR/THB spread in China, for example, will be wider than Thailand, but the actual value relies upon interest rates in the two countries, the volatility of the currency pair, and the prevailing conditions in the domestic markets in China. Plus MYR is a so-called 'exotic' currency, which means there are only a few houses willing to even deal it, much less publish prices. So it's an unknown, and in those cases the best predictor of what the spread will be tomorrow is what it is today.
Mark Mayo wrote that you can look at what the MYR/THB spread is for other venues simply by getting a newspaper or looking on the net. That will tell you if MYR/THB is more liquid in Laos, or China, or Burma. Also you can look at research from one of the large houses that publish research. But remember that whenever a bank discovers that it can purchase THB in China and sell it in Thailand for a profit (venue arbitrage), they execute it through their branch offices and the opportunity only lasts for a few moments.
Beyond that if you want to try something more sophisticated, you can take your question to https://quant.stackexchange.com/questions. The tags would be 'fx' https://quant.stackexchange.com/questions/tagged/fx and 'exotics' https://quant.stackexchange.com/questions/tagged/exotics