What are they looking to do? a legal marriage or they just want a budist ceremony? one is legal one is not.
Where do they want to do this Bangkok? an Island resort? in a village? Do they know anyone in Thailand? If not I would suggest contacting Seanthe Pilot on here. He is a canadian lives in Phuket.
If they are staying at a resort the staff there could arrange everything.
If they are going to Bangkok I know someone that could make whatever they want happen.
Regarding costs well depends on where and what they want.
A little more information would be helpful.