I think it all depends upon how long you plan to keep your trailer.
Remember, dealers don't cover them because they plan to get rid of them within the year. If you plan on keeping your trailer for several years, then storing inside can be worth the money.
We store ours over winter at the county fairgrounds in an enclosed building with concrete floors. We pay about $350 and feel this is a good expenditure to keep the trailer out of the weather (any weather) for a good period of the year. No wind, no snow, no rain, no sun. During the use season, when not camping we just park it in our driveway, but if I had a large enough pole building at home, I'd store it in there when not camping.
It only makes sense that the more protection from the elements that you can provide to your trailer, the better and longer it will last.