I don't know much about WSS itself (we're looking at MOSS) and I'm not a developer, but that strikes me as a pretty good question.
If you can develop your own solution using WSS cheaper than the MOSS licence cost, then perhaps that's the way to go. But you'd need to include all the costs of planning, developing and supporting your product, plus the cost of licensing any other platforms/products your solution uses. Then compare that against the full licence/support cost for MOSS.
In our environment I imagine that MOSS route will end up cheaper overall, given that MOSS seems to offer most of the functionality we need with just a bit of configuration and no development. But that's not to say in-house dev isn't better for someone else.
(ouch, I got a splinter from the fence).