Actually the filter is probably one of the crystals on the board. Dual conversion RXs have two crystals in addition to the one that you plug in. And they are fairly fragile compred to much of the rest of the RX. If they crack, the results can be hard to notice on the bench, but might only show up on the ground with engine vibration. And even then, it might be sporatic. I've had a couple of RXs do that over the years. As for the cost of those extra crystals, I've no clue. I doubt they are very expensive, but as you said, it's not the parts count that matters, it's the labor