Sin sot and party costs usually reflects the status of the families – including the number of invited guest – so, you wife shall have good knowledge about it.
Average costs really depends on status; so, average cost in which level. A poor couple's wedding might be a very modest reception, where the money guests bring pays for it all and more (I've experienced that). Higher social level weddings might be with a guest list for hundreds – or even a thousand or more (I've also experienced that) – guests; so, we talk about several millions of baht. In that view, 500,000 baht is not out of range.
Dowry is a question of negotiation. Also, if part of the sin sot – or all of it – shall stay with the new married couple.
I'm not sure if "Thailand is cheap", especially not when it's higher social level weddings. I've been invited guest at four weddings in those levels, and they didn't appear cheap at all.