BOM cost of a basic XLR output derived from an output buffer is, well, the XLR socket and two (2) equal break-out resistors, 100Ohms typically. One goes to the buffer, the other to ground. This fulfills the balanced connection requirement of that there shall be impedance symmetry. Signal symmetry (wrt ground) is NOT required to exploit the virtues of a balanced connection, but impedance symmetry is.
This kind of XLR output of course has the same signal level of the corresponding unbalanced output (derived from the buffer output with a single break-out resistor, again 100Ohms typically).
In most consumer gear, the balanced out is often made with a unity-gain inverter tapped off from the unbalanced output (the actual buffer output, that is) and hence offers signal symmetry and twice the total signal level. Sometimes, the level of the XLR out is made the same as the RCA output, at increased complexity.
In yet other cases, when the signal path of the device is fully balanced internally, we already have the signal-symmetric balanced output and then the unbalanced output is made from a simple differential amplifier of unity-gain (hence same signal level) or gain on 0.5x (half the level, to avoid headroom issues -- clipping) of that gain stage. Again, the break-out resistors are used, they provide isolation of capacitive loads (long cables) so that the op-amps used won't oscillate.