I have always asked that question myself.
Well I think the prices for arrangers are so high because they are made generally for the home player wanting to amuse themselves and a few friends, and not a professional market. That is why for example they go half way on the features on a new arranger. The manufacturers know that it is not important to go all out on the features because the home user will buy the arranger even if the features only go half of the way. It is only with in recent times that arrangers are being used professionally, but manufactures to me do not want to acknowledge that and make their arrangers accordingly.
Also, arranger sales are not at the same amount as synth workstations so they must price arrangers high.
They know that the high price they are charging for arrangers is not a problem to whom they are marketing and selling arrangers i.e. the hone player and not the professional market.
That�s why some manufacturers are slow to have 76 key arrangers, arranger nodules, samplers and full sequencers.
I think the prices for arrangers are too high. Could you imagine the Triton extreme (61 keys with no add-ons) costing around $3500 u.s. Would a lot of people buy it at that price?
It is only we the buyers can get a change by not buying new and very expencive arrangers with very few add-ons.
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TTG