So this is a site that you don’t control/own and you scraped? Why don’t you have access to the database?
It’s hard to say without knowing more about what the data look like. Is the site online somewhere?
Anything is possible. Do you want users imported too? Do you have email addresses? Are there user profile files?
How many topics and posts are there? Is there one topic per html file? One post per file?
For a frame of reference, I’d not consider such a job for less than $5000. And I it’s not likely to be pretty when it’s done.