Eyefinity is not a replacement for 2560*1600 unless you are talking about desktop work, even in desktop there are differences.
In desktop applications like photoshop, you really don't want to muck around across those bezels and perhaps different monitors due to color variations. Still eyefinity lacks vertical pixels so you still need to scroll up and down for large images.
For web browsing I would say 2560*1600 is better since many web pages, even this one is designed to fit 1024 pixel monitors. Extra horizontal pixels isn't doing that much good compared to vertical pixels so you can see more of the page at a time.
Now for gaming, 2560*1600 makes the image sharper and more clear. Think of a 2x zoom with the same field of view. So when you look down a sniper scope and things would look twice as big.
Eyefinity for gaming doesn't increase image sharpness and clarity. It would be no zoom but larger field of view around the sides. Things won't be easier to shoot but it would be harder to get ninjaed from the side. The support for eyefinity resolutions at this moment is pretty bad. Not many games support it and I don't forsee many games will support it because it is unfair.
As for price. It really depends on what type of monitor you are going for. All 30" 2560*1600 monitors are top end IPS or MVA panels which has much better color fidelity and viewing angle good for professional work. It is also great for movies and games since IPS panels can display up to 16.7 million true color. At 30" large viewing angle is a must otherwise you get color distortion around the edges.
Cheap 24" monitors on the other hand are TN panels. Not really good for anything except they are cheapt to manufacture. They are limited to a 256k color pallet so they must mix colors to simulate a 16.7 million color pallet. The difference is really pronounced if you compare to a IPS panel. TN panels have dreadful viewing angle, but I guess it would be fine if you tilted the eyefinity monitors.
If you buy multiple 24" IPS panels, they would amount to a similar price as a 30" IPS panel so it really isn't all that expensive.