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Greetings. As for the cost of Hauptwerk itself, you can download a free version with some limitations, but it'll help you make significant headway in deciding whether to buy the software (which costs about US$250 for the basic version). The free version includes a nice organ with plenty of stops to set the feel, and you can download a number of other free sampled organs from various sites... still, no cost, so far.
Rather than Hauptwerk, you might also explore GrandOrgue - which is 100% free since it's open source freeware.
But now, as for the rest of your set-up, that's harder to do on the cheap...
You'll need some kind of MIDI-enabled keybed (one or two manuals?), plus you'll likely want a MIDI-capable pedal board to take advantage of the pipe organ pedals.
Regardless, without a computer you won't get anywhere at all, since you need one to run the Hauptwerk software (or GrandOrgue) itself.
There are plenty of posts throughout this forum here (and on the Hauptwerk site) about what kinds of gear you'll need to get up and running, so I won't attempt to replicate these (nor would I do a very good job since I'm a relative newbie at this stuff).
You might also check out www.pcorgan.com where there's a wealth of info relevant to your question(s).
Happy hunting - OneWatt