I Got Married in Thailand in 2011, The Amount was up to me, So as I had been to my Wife's Sisters wedding a Yr or So before, I used the Amount Given by her husband as my Guide. It was About 250,000 Baht, I think. But, I don't think there is any Set or Expected Amount. I have also seen Cases where the Amount Given is Given back to the Bride & Groom as a Wedding Gift. So, It all depends on various things.
Also depends on if your seen as a Walking ATM. ???.