Truth is,whenever to order anytning a la cart,and I mean ANYTHING,it gets expensive.They keep costs down,by putting together drums in a package,.assembly line style.When you deviate from that,it costs them more money,to actually build an extra bass drum start to finish,and ship it.
Like I said ,the same is true for just about everything.Try ordering a custom guitar finish from Gibson,and see what it costs you.Go into a restaurant,and order a la cart ,as oppose to a packaged meal,and see how much more you pay.
Its not just Tama,buying by the piece is going to cost more,no matter.
I was going to buy a second, smaller bass for my Stage Customs. It would have been $319 (USD). To get the Bop kit, which comes with that same bass, a 12" mounted tom, a 14" floor tom, and a tom mount, it was $499. I went with the Bop kit.
In other cases, where you might have to buy a much larger set, at a much greater cost, it probably wouldn't make sense to go that way. And in some cases, the individual piece you're looking for might only be available that way.
Got to weigh the pros and cons, but individual pieces are always going to be more expensive than getting them included in a packaged set.