I think one reason for the difficulty for teams to enter F1 is that there aren't other series where each team builds their own chassis. It's difficult for a privateer team to build an F1 car, HRT had a Dallara-designed chassis, Caterham bought the old Arrows factory, and Super Aguri started with old Arrows cars. Another series with multiple constructors (I don't mean F3-style with teams buying chassis from constructors) would make the new F1 entrants more ready to be F1 constructors.
But I think F3 is the only feeder series with multiple chassis constructors and teams just buy the chassis. Otherwise all feeder series are spec series. Open formula e.g. in GP2 would increase the costs so obviously it wouldn't be feasible. But I'm afraid the number of F1 constructors will only decrease. Will F1 end up being a series of a handful of constructors and the grid is made full by three-car teams or customer cars? In the current financial situation, I'm afraid F1 will lose manufacturers, and getting new ones would be hard.