Your entrance fee helps pay for all the costumed performers (the "living history"). The ones I saw were all well-prepared and entertaining. If you spend just two hours - you will only briefly hear some of them. The fort is pretty big too and has good displays. It takes a while just to walk around. You are right by Lake Ontario and the mouth of the Niagara river which provide excellent scenery. I think I would plan on four hours - although it obviously can be done in much less.

For 2018 I believe the rate is $11.70 for adults and free for children to age 17

$12 per adult. Seniors or AAA get $1 off.

My answer may be too late, but this year (2017) is Canada's 150th birthday so many of their parks are free entrance. We didn't pay an entrance fee, just $6 Canadian to park. It's not that far to drive from Niagara Falls. I think you could enjoy it a bit. I'm just not sure how late the people at the site are there doing re-enactments.