Well its not just WL, all portable blues are expensive. Look at all the 15mW blue portables and you'll notice the prices are often more than the cost of a 50mW+ module. Greens are similar though, just not to such an extent.
On the low end a pen portable is actually cheaper sometimes (with greens) because it doesnt require all the overhead parts like cooling, AC-DC PSU, etc. As you go up in power though it becomes harder and harder to make it portable though, and then the prices of modules start killing portables (I could get a 300mW module for well under half what I paid for my RPL 300).
Blues are similar, but they haven't really caught on in portable form - very little demand - so even portables are insanely expensive and demand better cooling than with a green portable. And with the few limited portable blues companies have an excuse to charge more. You may have noticed though that in the past year portable blues have been coming down in price on the low end whereas modules haven't fallen anywhere near as much.
Eventually portable blues would have gone the way of the greenie and we'd be seeing IIIA ones <$200 eventually... EXCEPT that now it looks like 440nm diodes may enter the market in the next few months/years. I don't really know how that will play out, but it'll definitely be interesting to see how they compete with blue DPSS in the <50mW area.
Sorry... I guess I haven't really answered the question... just rambled. Anyways though, remember that its not what portability and waterproofing cost to make, its what a company can get people to pay for it. Right now blue portables in general are expensive because its a relatively new innovation, and there's no significant demand bringing the cost of the innvation down, so WL can probably get away with selling a few top-dollar blues.
At least with the blue modules, lasershows and scientific apps keep them produced in reasonable quantity. With the green portables, there's enough of us power hungry freaks to buy up a bunch of IIIB pens. With portable blues though, you're gunna have an overhead cost to designing and starting production on them, and there's little demand to cover those costs so prices are relatively high on them.
Or... at least that's my best guess for it. I'm sure the exact explanation is a much more tangled and complicated web of factors.