Installing an orange box, in and of itself, in an aircraft, is probably cost manageable. Buying the box? Not my area of expertise. However, with present technology for legacy aircraft systems, connecting the orange box to the aircraft systems for data gathering could become very costly. Of course, for any operator who considers it worthwhile, nothing prevents undertaking to work voluntarily as a modification.
Enabling investigation ease with the provision of FDR data post accident is one thing, perhaps devoting that large cost per aircraft to pilot refresher training instead would be more beneficial as being preventative. For those few "mystery" cause all fatal accidents there is likely common agreement that increased pilot skill and recency might have prevented, or at least minimized the severity of the event.
At present, TC Aircraft Certification branch cannot keep up with the demand for modification design approvals. Suddenly requiring a large group of aircraft to be modified with and FDR system would completely overwhelm the industry - and TC Aircraft Certification branch knows this. It's nice for the TSB to float the idea, but even they should be able to understand that it cannot be accomplished.
As the costs to fly increase, pilots will fly less. New pilots will not be able to afford to accumulate the experience we once did. So I guess its a good thing that at least some of these lesser experienced pilots entering the commercial world may go into the right seat of aircraft for which an FDR is already installed.