When I lived in Seattle, I was blown away at the cost of storage... for indoor storage with electricity, in my search, it was near impossible to find anything for under $250-300/mo. The public storage places in Bellevue and Issaquah or anywhere near water wanted even more... and that was 5-6 years ago! I ended up storing the boat in the biggest POS storage facility ever... 30-40 minutes away in Auburn, for $140/mo. I still felt like that was ridiculous, but compared to Bellevue/Issaquah, it was a bargain. I bring this up, because it's all a matter of priorities, convenience, and disposable income. Problem is, so many around Lake Sammamish put a premium on convenience and have high disposable income.
I wouldn't think that Wakeworlders (defined by me as: individuals who are significantly more enthusiastic about watersports than the general population and make sacrifices in their personal lives in order to afford their chosen lifestyle) would be your primary market for high-end storage... My guess is your target market is the Microsoft-type weekend-warriors with plenty of disposable income, who just bought their first boat on impulse because it seems like a fun way to blow some cash (and yes, I am aware that makes me sound like a wakeboarding snob... so be it).
To answer your questions:
1. I don't fit your criteria of being a Lake Samm boater. But if I did live in Issaquah and boated on Lake Samm, I still would not... as I would find a house with storage or a garage that would fit my boat because that is my first priority. If I didn't have that option, I would get creative and make friends with somebody with a big-enough garage.
2. Per my post, reasonable is relative... if it is really nice and convenient, I would imagine you could get away with $300-400/mo. But like I said, I would never pay that.
3. I would want the basics -- power, a garden hose attachment nearby, access at ANY time, and surveilance (preferably live surveilance, but video cams are better than nothing). Additionally, if I were in that market, I would love to have the option of a monthly service add-on in which one could call ahead and have the boat completely ready to go, fueled and all... and cleaned and put away at the end of the day.
Good luck with your new venture, should you choose to pursue it!