Part of the problem is that it is expensive to hire singers to provide their voices for the software, and studios must recover the cost each time to make it worth producing the software. They must also gain a profit for their respective studio so that continue development as well as find funding for investment into new techniques and technologies that can aid towards newer Vocaloids. Obviously, as time goes by prices will come down as more consumers buy the software. Older versions are in not so high demand as newer and are likely to be overall cheaper depending on circumstances with the studio who produces the software.
Years ago music media based software was so expensive, that only top end studios could afford to buy the software. Over time, more and more consumers have used and purchased the software, causing the prices of all music based programs to come down in price. The same goes with Vocaloid, as time goes by the software will come down in price with an increase overall in sales revenue.