If you go all out with all the bells and whistles (high end computer, 3 screen setup, 8 contactors for force feedback, LED flashers, shaker motor, decals) you could easily be looking at $3,500. If you cut back you could probably get the cost down to $2,500 with just the cabinet (if you build it yourself or find a donor), 3 screens and nice computer and still have a decent setup. Having built one myself, my advice would be to not skimp on the computer and video cards. The tables are so much more graphic and resource intense then they used to be and the last thing you want is stuttering during gameplay.

Any good sites / info on building one ??

Of course. VP forums is the place to be.
Here is link direct to the Cabinet builders thread.
Tons of threads on how people built their cabs with lots of photos.
http://www.vpforums.org/index.php?showforum=29

The problem is that it would occupy the same space as a real pinball .I would rather have the worst pinball ever made than something fake.

Building yourself....

I would say you can go 4k+ for a nice 3 monitor setup all the way down to the $1700 range for a 2 monitor mini-pin.... It really depends on what you put into it. Don't go cheap on the CPU or the Monitors - get what you are gonna keep - they are basically impossible to upgrade later. I got 4k+ in mine. I love it, and it's VERY valuable to me. Honestly, it's not the real thing, and I don't think that it would stay in a larger collection. In a smaller collection of 1 to 4 machines, it's great. You can get the "feel" of playing 100+ HQ games in a smaller collection, and even thought it's not real, it is still awesome pinball fun. Obviously, I am biased, as I have made 5 (yes, I have did some serious upgrading). http://www.youtube.com/boogies2

Here's a link to the Hyperpin forums: http://www.hyperspin-fe.com/forum/forumdisplay.php?32-HyperPin-General-Discussion That is the best site IMO for researching how-to's on a cabinet.
That is the front-end software that launches the visual pinball / pinmame program. You can get tons updates through this site. I use the VPForums for keeping up-to-date with table that are WIP and newly released.

You DEFINATELY need to try a decent setup to decide if it's something that you would enjoy. If you have a High-End Gaming PC, you could probably run one of the tables as a test and see if it looks like something that you would enjoy. I would recommend trying all real pins in the same manner of "try before you buy", but with the Virtual Pinball setup, they are kinda far and few between, but the scene has definately been growing.

And that's you. I have 20 "real" pinballs and do have a lot of fun with my virtual pinball cabinet.

I have 8 real and also love my virtual cabinet.

Even though I have owned over 100 pins, My virtual pin is the only way I can play them all at once.
Everybody who has played mine thinks it is fantastic.
How many people have Big Bang Bar, Cactus Canyon, or Medieval Madness they can just walk into their game rooms and play when every they want.
Virtual cabinets are a great addition to the real things, they are not trying to replace them.