The local real estate companies in a city have joined together and arranged a "property fair." the expenses of the event will be shared equally by the sponsors. though many companies compete against each other, they have joined together because the medium will the companies market themselves through a dedicated forum at an extremely low cost. this arrangement is best referred to as

coopetition

Explanation:

Coopetition refers to competing firms cooperating together in order to gain mutually beneficial results but they still compete with each other. Coopetition is not collusion, since first of all, it is legal and ethical, and collusion tries to alter the market forces (e.g. set a price which is not the equilibrium price) by reducing competition.

By cooperating together, the real estate firms will be able to access a large number of potential customers at a relatively low cost, but even at that property fair, they will compete against each other.