Metal bonded laps have a thin layer of diamond, usually electroplated in place with nickel plating. With use, the plated thin layer breaks down and starts cutting like a finer grit. Sintered laps, on the other hand, have a very thick (3-6 mm) cutting matrix. This contributes to a much longer life and steady, aggressive cutting performance.
Although no exact statistics exist on the long-term economics of metal bonded versus sintered laps, you can probably count on at least ten times more cutting effective cutting time from the more expensive sintered lap. For many, a sintered lap can easily provide a lifetime of steady, reliable performance. At a cost only 3-4 times higher than the bonded variety, choosing a sintered Lap makes good financial sense, but yes, that purchase feels, and is, expensive.