So, one hex of a rowhouse has been rubbled.
By B23.71, the black bar between the rubbled hex and the adjacent rowhouse hex is still there.
But how does movement between those two hexes work? B23.71 just says three MF to bypass the black bar from one rowhouse hex to another. It doesn't say anything about three MF plus one for going into rubble.
In fact, it only specifies "Infantry in a Rowhouse may move/rout/advance/Withdraw-from-CC directly into another Rowhouse hex...", but doesn't say anything about a former rowhouse hexes with the black bar still in place.
No, it doesn't say anything like that, but it doesn't have to. What it does say is that you can move around the black bar using "bypass" movement (note that it's in quotes; it's not "real" bypass in the normal sense of the rules). It specifies that this "bypass" costs 1 MF, expended in the hex that the unit is leaving. The MF cost to then enter a rubble location is 3 (B24.4). So total MF cost is 4.