My approach would be to combine a Linux SBC (such as Raspberry Pi - can be had for around $25) with a coin slot mechanism (again, around $25), wifi module + antennas and a robust housing.
I'm not sure whether you're looking for a design, a prototype or a complete implementation - I've priced based on design + software work only.
Could you clarify whether the coin mechanism will provide access to everyone in the area, or just per-person? Either is possible with the above approach, it's just a matter of software configuration.