That is totally up to you honestly.
Usually when a user wants something custom like this that would fall under custom development.
I would suggest if you really want reviewpost to wait for your ability to pay for a custom module. What I would do is email me at email@example.com
so we can work on getting a more accurate quote for you. The range as stated all depends on the amount of work needed to integrate things. If burning board only needs user authentication and no style integration then you would most likely be on the low end of things.
It is not recommended to use Reviewpost as an internal install if you intend to switch later as it would be hard to match userid's later and require extra custom work to realign users.