Used parts are not worth 50% of new, 60% of new, or any percentage of new. If you sell them based on formulas like this, you are short selling yourself.
We live in a capitalistic society. The ONLY factors that determine price are supply and demand. That is, the value of anything is based on that of which someone will pay for it. NADA can say your jet ski is worth $5000, but if it sits for a year unsold, NADA is wrong. The marketable value of that ski maybe $2000, and since marketable value is the only thing that counts (unless you sell insurance), your ski is worth $2000. It works the opposite too, my quad is valued at $1300, but I could sell it for $1500 in a heart beat (people are begging for it at this price). It is worth $1500, or more.
It's easy to explain the cost of used parts. Used parts are worth what ever people are willing to pay for them (and what people are willing to sell them for, despiration is powerful). The values of used parts are so high because they do not depriciate. Our sport is one designed arround preformance in a corrosive environment. All of our parts are made out of stainless steel and aluminum. Just because the ride plate is used, doesn't mean it's worth just 50% of new. It's functionally and durably is as good as new, it's simply worth less than new because it's not shiny and "new". An aluminum head will bring $40 less than a new one for the simple fact that there is no difference b/t the two. It's aluminum, and the odds of the item having a pre-existing condition at the time of purchase that is not apperent is highly unlikely. Therefor, the risk to the second hand buyer is very low, so the value of the item remains high. The only reason used parts are worth less than new, is because noone will buy used when the can buy new.
The prices of new have to do with three important factors, supply, demand, and cost. If demand doesn't justify the cost, there will be no supply. Costs rise to offset demand. It's simply not the same as used.
As for the comparision of the stand up community VS the MX community, well, there is no comparision. There are a number of contributing factors, the two most previlent being the number of riders and the technology.
MX has FAR more riders than JetMX (do we have an abbriviation? I'm dubbing us JMX!!!). There is a MUCH larger community purchasing MX than JMX. That means there's more dollars in the sport and more hands reaching out for product. The discussion has been about aftermarket, but think more simply at about OEM. There's a dozen major manufacturers of MX, but only two are involved in JMX. There's simply more people.
The biggest topic of discussion is about purchasing new aftermarket production. Purchasing new aftermarket parts is not only going to have no impact on the sport, but it's a waste of money. There is NO comparision at all between JMX and MX aftermarkets. Everyyear, for the past 20 years, MX has seen HUGE improvement in technology. Each and every year, the model you purchase is better than the year prior. Noone will tell you to go out and buy a 1996 anything to take to the track. It doesn't happen because the techknology in MX improves every day. The suspensions improve, the engines improve, the everything improves. There's not one part on an older bike you'd take off and put on a new one. People go buy a 2008 MX pipe because it is different and better than the 2007 MX pipe.
How many times have you heard someone say "just buy a 1996 roundie, it's the same as the 2007". Why go buy a brand new B-pipe when it is exactly the same as the one made 10 years ago? THe aftermarket preduction doesn't suck because people don't buy new, it sucks because new is excatly the same as used. With the lack of new technology being introduced, coupled with the fact that fiberglass, stainless steel and aluminum do not (not really) deteriorate, there's no reason to buy new.
The lack of new developments in the sport, coupled with the low number of players, makes the market suffer. There are people who buy new, but most buy used because it's the same, and since there's so few mouths to feed, manufactures are not wasting their money to bring food to our table.
Two things will bring more new product and lower prices to the sport. More people, and more technology (making a reason to actually buy something new).
(Used buying guides base their prices on the idea that noone would pay more for a 1996 than they would for a 1997 all things equal, but they do not take into consideration the avalibility of items. Given the 1997 is not avalible, the 1996 may sell for more than the 1997 sells for at a different time, and in our sport, the superjet is the same.)