Well, I have invested more than I can imagine and I am just putting the observatory together now.
To make it really attractive, you need a big scope on great mount. I say a big RC or newt (abt $10 -$15K and up) on a PME II (abt $25k esp if you want to go remote) with a good camera and accessories (abt $5k and way up) would be a pretty good starting point.
I'm building a 4m roll-off atm and thats probably $5k+ when done.
Straw on the floor is enough for creature comforts if the scope meets the mark.
Anyhow... thats one ball park opinion..