While a sprung solo seat can be DIY'ed for less, I've looked at the various components online and Mike's price is not out of line for what he's offering (and for those who can't DIY, it's the ONLY bolt-in option). There are seats cheaper than LePera and there are WELD-IN brackets for $50.
Once again (directed at no one in particular), if you can make it yourself, shaddap and go do it. Otherwise, go for a ride.
