Fuel Tank Breather Valve 13901711395 is used on E24 635CSi, E30: 318i M42, 318is M42, 325i M20, 325is M20, 325ix M20, E31: 840Ci M60, 850Ci M70, 850CSi S70, E32: 735i/L M30, 740i/L M60,750i/L M70, E34: 530i/540 M60, E38:740i/L.

The price is extremely expensive with MSRP $452.84 , a bit cheaper when you by it from the open market as Bosch part, BMW 13901711395 is Bosch Nr. 0 280 142 150 .
But there is an alternative:
The valve with BMW No. 13901465030 costs MSRP $72.53 from BMW. This is installed in newer BMW motorcycles. You can get it on the open market for around $ 30. BMW 13901465030 = Bosch No. 0 280 142 310

However, there are 2 small differences that should be noted:

On one side the new valve on the connection side to the activated carbon filter has an 8mm connection. The old valve has a 6mm connection. The other side (towards the suction) is identical with both 8mm.
Exchange the 6mm hose for an 8mm hose and also attach it to the activated carbon filter on the 6mm nozzle. This is completely easy.

On the other side you have to turn the rubber ring from the holder/bracket so that the flow direction of the new valve is correct (installation position is marked with an arrow).
This tip was posted on the German 7 Forum.