According to Wikipedia, the bike trail gets 175,000 - 200.000 peole annually. I'm not sure how many would go to KI from the trail, but I would assume bike trail population would increase with a KI entrance. If it increased by 50,000, and 10% of the bike trail users went to the new KI bike trail entrance, then we would be talking about 22,500 to 25,000 guests annually going through the KI Little Miami Bike Trail entrance. If you throw in train riders going to and from Great Wolf Lodge (maybe 500 per day), Boomerang Bay, and just riding the train for fun in addition to the 1.3 million that currently ride the train annually, I could see 2 to 3 million riders on that train in the future with about 50,000 of those being new KI customers due to the Little Miami trail and Great Wolf Lodge connections. Estimating excavation and construction costs is much more compliacated, but 50,000 customers at $85/ pass could be $4.25 million, but many of the non-KI pass holders that go on the trail (175,000-200,000) annually will just stop by the Little Miami entrance for concessions which could also be factored in.
Here are some references:
http://en.wikipedia.org/wiki/Loveland_Bike_Trail
http://www2.paramountparks.com/kingsisland...l.cfm?ai_id=151