DMT D8C $50.
Amount I spent on sand paper prior to getting the D8C - a whole lot more than $50.
That stuff ain't as cheap as it used to be, so how cost effective it is really depends on how many stones and of what type youll need lapped. YMMV.
Also CKTG has a 400/1K 2 sided diamond plate for something like 30 bucks. I've been meaning to pick one up, that's a heck of a deal.