when i had a trailer it was £5 a week to park at yard and it def was not secure as the trailer was stolen!! i now have a 7.5t lorry which i pay same for at another yard and for 8 months of year it is locked in a grain barn!!
I keep my trailer at the yard and it's included in my livery.
We used to keep a caravan at a local storage place which had very high security and it cost £200 per year. However, it can vary greatly in diff areas. My parents pay £400 per year to store their caravan in Bedfordshire. There were horse trailers at the caravan storage place we used - and boats too.