I paid under $10k for my trailer brand new in 2004.  I maintained it and enjoyed it for years, but I never covered it.  I can share photos of it under several feet of snow, and relate tales of the freeze/thaw cycles it's been through, etc.  It's never leaked or let me down in that regard.  
I've been planning on replacing it for a few years now, but just haven't gotten around to it.  It still suits my needs nicely.  However, the rubber roof was shedding the protective white coating, and showing its age.  I re-coated it a couple years ago, and am now in the practice of covering it in the winter.  
As far as I am concerned, even if I purchase a new cover every year, it's a good, even frugal, decision.