Nothing is VERY secure, but I got a wood framed shed kit from Costco (8x12) for about $1,000. Built that up with another $200 or so for supplies and it holds all my bikes quite nicely. It's nice having all the tools in there, all the bikes, spares etc. I didn't want a plastic or metal shed but those are cheaper / easier to install if you are inclined that way.
At some point i'll hang some of the bikes from the rafters but for now I have 6 bikes, 2 frames, extra wheels in there with plenty of extra room for other stuff.