I think it is only resonable to pay if you have hard standing, cover or secure location... most of us cannot keep our trailers/lorries at home and caravan storage also costs.
I would say between 50p - £2 a week for trailers if outside and space is limited and slightly more for lorries... with security maybe double and if under cover same again?
I'm DIY and keep my lorry on the yard for free. It's hardstanding, not concrete and there is plenty of room. I've been at yards where they have charged before, but I kept it at home then and can't remember how much it was. It was maybe £10 a month, or something like that?
Hardstanding, cctv, locked gates, out of sight to everyone unless you are at the yard, and so easy to get in and out of its space = FREE !
Last yard - it would sink in mud, you were trapped in by other cars, horseboxs and caravans, and you had to pay £5.00 per week - oh and if you tried to go out anywhere you had to inform the yard nosey where and when you would be back, else they would not move the car and if they were being particulary horrid and knew you were off to a show they would block you in and bugger off on a hack and leave a parked car in front of you !
Ours is £10 a week, understandably everyone leaves theres at the yard next door which is much more secure for trailers and only £5 a week. The only two with trailers on our yard are the yard managers as they get it for free!!
£2 per week, parked out of sight on the concrete yard and locked in at night. Lorries are £3 per week.
My previous yard charged £5 per week for a trailer and £10 per week for a lorry which I thought was extortionate - at that time we kept ours at home!
I don't pay anything to store my trailer at yard on hard standing. I've always wondered if people charge for this does it imply they have some sort of liability to the safety of the trailer/lorry from theft or damage?