Whether 40 percent is reasonable or not depends on what the host is providing and your expectations.
Remember, if the host is a bookstore, they will have to make some accommodations to host your event. That might include extra staff, longer hours, etc. So they may have additional costs that need to be covered, on top of their usual operating costs.
Regarding expectations, you should expect to split the sales with your host. If you're self-published, you probably are already doing a similar split with your self-publishing platform (such as Createspace), and a 40 percent take on those platforms would not be unheard of. So why should it be different in a real life situation?