i don't think you can come up with a magic number for all 6m ribs. as well as obviously depending on condition, how its been stored, how many hours are on the engine etc; it must to some extent depend on brand, like cars. i guess anything which makes your boat less appealing to the general buyer (e.g odd seat layouts, wierd colours) will make it harder to sell and probably worth less. a bad choice of engine might make the package seem less attractive/valuable too.
a while back i mentioned a "typical" number of 10% p.a. that someone else had suggested. and this was totally dismissed by owers of some of the more "quality" (or percieved quality) brands, as being was too agressive.
If a similar boat is available brand new quite quickly that will also make it depreciate quicker at the start. If the manufacturer has a 6 month lead time, then a 1 year old boat may be worth closer to the new price.
time of year makes a difference too (cheaper at end of season, than start of next)