I'm keeping an eye on prices as well... one place offered to build reinforced frame members on the sides so I can mount solar panels and Rotopax containers on a 6x8 with a ramp door and a side door with a RV type lock (so the door can be opened from the inside, and the trailer used as additional sleeping space in a pinch.) Heavy duty barrel bolts and a bar across the curb side door on the inside would ensure that when stored, the only way in would be the rear ramp door, while still allowing someone to get out.
Forgot how much they asked for... I'll go check, but it was about $3-4k, although the price they quoted me was higher than normal because I was asking for some specific, custom items, especially frame and axle related (I wanted a heavier duty axle and frame than the usual trailer so if I wanted to have a battery bank, I wouldn't be overloading it.)