I think the implementation of dual spec you are likely to see when the PTR goes live is going to have some substantial differences from the initial announcement. We tend to iterate a great deal, which is one of the reasons we don't do big announcements more often.
I don't think they will make it cost 5000g... Actually there were no blue post yet that names a number. I guess it will be something about 500g to 1000g.
They don't want it to be a premium feature that casuals can't access easily.
I would imagine that if they add more then 2 specs that additional specs would be really expensive.
Casuals can easily access hundreds of gold a day, it's not hard via dailies, not even stepping foot into a dungeon. In fact it's the casuals that buy up the BOE epics from Naxx and other dungeons at premium prices too ;D. They can afford a 500-1000g base (they did it to fly).
As a hybrid class, two is just the beginning... and very basic at that. My main is a priest and I run shadow. If I were to go heal, depending upon the fight deep holy is good, or on some disc is better (just PvE). That right there is three specs and doesn't even think of a slot for pvp. Now, if I were being cheap, I could probably settle on two or three. But if I had the option to buy more I would.
I think they could easily implement more than just two specs, and I think they might at least eventually (probably not right away). Maybe add them in like bank slots/gbank tabs with the more slots you have the more you pay. I think this will be an important feature that raiders as well as pvpers will need to evaluate and submit good feedback on. Now we get to wait for the PTR and other blue notes.