There are way too many variables to answer with one standard answer.  A lot depends on the type of trailer and it also depends on how the trailer is used and taken care of.  I think a living quarters trailer depreciates about like HWBar stated, more the first year or two, but then not much after that.

I think a steel trailer will depreciate more than an aluminum one will, especially if we're talking about one that is 10 years old or more.  But again, a lot of this depends on the type of care the trailer has had, and how heavily it has been used.  I had a 2003 Titan Renegade bought new in 2002, and sold it this spring on this site for within $300 of what we paid for it.  But it was not pulled a lot, and was well taken care of.

I am sure people have some scientific, well-proven, formula that involves many variables such as brand, height to width ratio, and air pressure in tires.

However, I would propose that annual depreciation is kinda like the price of a horse.  Both horses and trailers seem to me like it really depends on whether you are buying or selling. 

I agree with the previous posts as far as the first several years go,but at sometime after that(at least it appears,historically)that trailer value stabilizes and retains there origanal purchase price or at times even greater value than when purchased new.

Here is my example, about 7-8 years ago i bought a 2 horse bumper pull of unknown manufacture in reasonable condition that i estimated to be a mid seventies model for $875.00. Later on my cousin had a newspaper clipping from 1976 advertising a new ford car for $2,600.00. Now what do you think that trailer cost new in and around 1976? I'd guess less than $875.00! Tractors do the same thing whereas a 20 year old tractor in good condition will easily bring more than the origanal new sale price.

                                    Ardly