Why Aren't They Always All Varieties: Clash Vs (Crack, MPD, RPD, RPM, Etc)

Having a very hard time following what it is that you are really asking here. I will give it a shot to see if I can help a little. It appears you are referring to value. Specifically the differences in value. The value of anything is not finite. It is fluid based on supply and demand. In the error/variety world a lot of it is dependent on rareness and appearence. Simple clashes where a very small and very faint images are visible will be greatly different in value than a coin exhibitting full clash marks. One reason is because strong clash marks are a lot harder to find, and another reason is because strong clash marks are mucn more desireable.

As far as comparing value of errors vs. varieties, you really cannot compare the two because they are completely different things.

A variety is on the die before the first coin is struck. RPMs are misplaced punching of the mint mark. Doubled dies are mishubbing of the dies.
There are many different reasons for a doubled die to happen.
Hub alignment incorrect.
Using a different hub on one of the hubbings.
A hub that gets warped between one of the pressings into the die.
All these events happen before the first coin is struck. Die events such as polishing, cracks, chips, die scratches happen during the dies life. These events do not increase/decrease value of the coin. They are events that confirms different die states of the coins struck. On some dies the die wear is determined by the dies deterioration. On some varieties, this can reduce the value of the coin struck because some of the spread may not be seen on the coins because of die wear. Events happening to normal dies happen, but may mean nothing because they are not on a variety. These events are like fingerprints for a die. They do not confirm a certain die unless first the hub doubling/RPM, over date is one the die. If the variety is not present, then it is just a normal die with a chip/crack/die scratch that will mean nothing as a premium because the variety is not present.
On dies the same events happen over and over on the striking process. The same clashes happen, the same polishing happens, die cracks appear, die chips show the dies breakdown, without the variety, this will mean nothing for a normal die to have this event. Breakdown of the dies is common. Eventually an event will happen that will retire a die and it can no longer be used. So a lot of the events that happen to most dies are interesting to new collectors, they are old hat after looking at them for years because of the large number of die breakdowns on normal dies. The markers are just handy for variety identification of a die state or a die stages listed by some attributors.

Mdpmedia, there is a market for varieties and errors. Both are sold on ebay and other places everyday. Some people collect RPM's, others collect BIE's, etc. The strong Doubled Dies probably command the highest prices, Die Cracks probably the lowest, but there are plenty in between for collectors at all levels - even collectors of one of a kind coins. It's all up to the preference of the individual.
As far as values go, there are numerous "pricing guides", of varying opinions. In fairness, the reputable ones vary slightly. They arrive at their values, by considerations of supply, condition, demand, previously sold for, and other factors. Keep in mind these are "guides". The ultimate value of a particular coin is set by the owner of the coin. The owner sets the value at what he is willing to sell it for. If it sells, that's the current value of his coin. If it doesn't sell, the owner has in essence, "bought" it himself. The value of each coin is set by the owner, whether he uses a price guide or not. It's all up to the preference of the individual.