For this i have messaged you privately that why this would cost you this much and why Xenforo is half the price of it. As I am not going to explain it here as this forum is not related to custom services work.
For the addon that you are looking for i was just checking and seems like this addon match with your requirements and cost you less.
Need an incentive to increase forum activity? Make your forum more fun for your users, let them show off how active/engaged they are. The more fun your forum is, the more your users will want to keep coming back for more. No one likes a boring...
In case you need some modifications in it you can always ask for the custom services for that or contact the author for that.