I've never merged forum databases to boost stats and I very likely wouldn't.
I have no idea what a fair price for a forum would be. Most people aren't exactly going to want to let go of their forums for very cheap either. I mean they worked hard to build them up so they don't want that work to go for nothing.
From a technical standpoint, merging databases is a pain and there is always a lot of room for errors to crop up. Plus it means downtime for your forum/site while the new database is installed. (And you want to only have downtime only when you absolutely can't avoid it. Like if your host has to do maintenance or if your host goes off due to poor weather or etc...)
And if you're buying active forums that opens up a whole other bout of problems. Supposing you merged sites just fine, you now have a lot of extra users who may not even stick around. Like discussed in
this topic. Which can make your forum look even worse if all these members with sizeable post counts vanish.
Overall if you want to improve your forum's stats, just keep:
- Posting.
- Getting packages done.
- Doing post exchanges.
- Advertising.
- Looking into new methods/ways to advertise.