Early on I looked at diodes as one big group... BUT that is not the way it works- mW and SAME nm is about as close as we can get.
There are many factors that dictate prices (& as you know--how big is the order)
We do not really have ANY 'buying power'--of all who use lasers== we are the smallest group and afaik none are made with US in mind.
Contacting places like Nichia on our behalf is a waste of our time. Prices change as new devices evolve for large volume users --
For instance-- yellow diodes and yellow DPSS- UNTIL some mass produced item comes along and makers are now in need many millions of them instead of hundreds will cost change much. That may be something we will never see on our lifetime .
Comparing lasers with fruit-- at the market we see many 'types' of apples, oranges and lemons- And prices are varied-- common lemons are much cheaper that, say, Ponderosa Lemons (thick skinned and used for making 'candied' Lemon Peel) or Blood Oranges- are much higher priced than 'ordinary ' common fruit. Seeded oranges are cheaper than navel oranges (seedless) -and blood oranges cost more than navels.
and... so that is your answer. Even within that same or close to the same wavelength cost varies. ..AND what we want means little to diode makers.
I do not know what the first 532 dpss were used for. my first green pointer was >10X what we pay now and before that even more costly .. and now green pointers are for sure the largest use. BUT us 'users' still do not have much influence on makers.
The pointer makers are the ones with the actual buying power, not us.
I remember a question-- "why are 650 reds so much cheaper than 635/638 reds ( of the same output)? .' Its the demand. And as we have seen-- prices on the 'brighter reds' has dropped some. So the demand has made these now made them cheaper.
see this pic from Norty-

can you tell which PJ has red diode with 2X the mW?
I can guess ...but it is only guess. They look the same to me.
also cost is NOT even close to the 'same'(but NOT 2X) -- but one is only 10nm more than the other. go figure.