I think there may be many answers to this question. First, mass-producing arranger keyboards for the home market and selling them in larger quantities than pro keyboards probably contributes to a lower cost per unit. Pro musos account for a small minority of the overall music market still. Second, beside being made in less quantities, pro keyboards have to be advertised heavily in pro publications/trade shows/endorsement deals etc. and require the most in R&D which drives the unit costs up. And, bands don't use arranger keyboards so even pro-level arrangers don't get the limelight that the Tritons etc do. That contributes to their misperception.

However, many pro keyboardists I talk to want to have the option of being able to do a solo gig as long as they can keep the quality of the instruments that they have become accustomed to, so I think there is a growing pro market for the cross-over arrangers such as the PSR2000, 9000 Pro, Tyros etc. There is still a minority of keyboardists who believe "arranger" denotes "toy", but that's by and large by those who haven't played an arranger lately. Likewise, I get people at my gigs who have low-grade arrangers at their homes and after hearing my 9000 Pro are motivated to step up to a pro-level instrument.