For the digital system I have at the moment, about $1300 (consists of K20d, DA* 55, DA* 50-135, AF500ftz).
I have spent much more than that, at least $6000 in total, but have sold much of it and gotten a good sum of the spent money back. I also have a few items unaccounted for because they are on the "For sale" list but I haven't gotten around to listing them yet.
See, what I like about this hobby is that you can purchase something used for a good price, put a little mileage on it, and then after you have played around with it and started lusting over something new, you can sell it again and not lose much on it.
Keeps money spent from LBA in check.