So, a couple of things: One, Arai is pretty much the only manufacturer that still uses sideplates on their visors. Keep that in mind, as it complicates visor changes. Second, neither the top Arai nor the top Shoei got the best ratings in the most recent helmet test I could find. Now that EPS liners are common, and well-researched, the polycarbonate helmets are stronger and more durable. The money you pay for a top-level Shoei or Arai (or AGV or X-lite or Suomi etc etc) goes to the exotic shell materials they use to reduce weight. For the average street rider, this is not likely to be a problem.
The most recent helmet test (anvil drops, recreation of the Snell curved-surface test, etc) showed the Z1R to have the best protection out there. It provided easily the best G-force reduction of any helmet out there, and the shell proved much tougher than the more expensive helmets. Many of the top helmets actually did worse, because their exotic shells are much stiffer than the plastic shells of the less expensive helmet brands, so they didn't provide as much G-force dissipation. More of the force was translated directly to the internal liner without being spread out first, so the EPS had to do more of the absorption. Many of the more expensive helmets used stiffer EPS too, which exacerbated the problem. The overall stiffer materials are lighter, but they are slightly less capable at dissipating impact. You can get a very good helmet for under $200 these days. Icon, Z1R, HJC and many other brands are perfectly good helmets.
Now, that all said, keep in mind that a plastic-shell helmet will not show damage nearly as well as the laminate shells, so you have to be more careful about banging it around. Any large impact (even a drop from hand height to a hard floor) will compromise the EPS liner, and it won't be able to protect your head anymore. Helmets should also never be stored in the garage where they can be exposed to volatile fumes from paint, paint thinner, gasoline, cleaners, etc. And, lastly, helmets should be replaced about every three-four years, as the EPS liner hardens over time, especially in the ultra-dry and hot climate you live in. You might be able to get more time from a more expensive helmet, but I wouldn't push it. Remember, no matter what you pay, it'll still be cheaper than a new head.
OK, rant off.