“What does a website cost?” is the same question as “What does a car cost?”. You can pick up some used beat up old pickup truck for $800.-, or buy a super exclusive Bentley limited edition for $2,000,000.-
I think indeed the most complex and costly element here is likely the transfer of the old forum content and port it over to the new forum. Not even sure if they’ll be able properly align the (most likely new logins and) new accounts with the old postings.