Just going by the fact that I've seen a lot of other complex cut/machined billet pieces for a lot less over time in the motorcycle and snowmobile accessory fields. Perhaps those other peoples' machines aren't running 100% of the time so the cost to use them isn't $150/hr lost profit.
In any event, regardless of what it actually cost you to make it, $289 is more than I'd spend for two pieces of machined aluminum as speaker grills, which is all that really matters to me. If you can sell a ton of them at that price then good for you! Must be doing something right.
2012 Victory Cross Country - Imperial Blue
2008 Honda CBR1000RR - Winning Red
2003 Honda VTX 1800C - Illusion Blue