CNN ran a piece about 4 weeks ago regarding an existing design. During the discussion, they quoted an installed cost of $100,000 to retrofit one plane. They went on to say the cost was minimal compared to the $1,000,000/plane cost to install Wi-Fi.
100,000 was to retrofit a cloud streaming black box, which have already been designed and are in service, not for detachable black boxes. Detachable black boxes would be much more (structural changes, test, cert, etc), not to mention the host of new failure modes it would introduce (can't wait to run over one of those fod's on the runway....)