Taxi from airport to Kepala Batas, RM$80. 

Both places are at mainland side, might as well take bus to Butterworth instead of flying into Penang airport..  If you want public transport, from the airport, need to take bus to the jetty (Weld Quay), take ferry to Butterworth, then bus to Kepala Batas.

Take Rapid Penang 401a to Jetty at Georgetown. Then use the ferry to Butterworth mainland. From thereafter go down from the harbour, you need to go to the new Penang Sentral Bus Station. You cna see it from the ferry.

Then ask around the Sentral bus station have some bus non air con to Kepala Batas.

2nd option is, after take bus 401a drop at Sungai Nibong Bus Station. You can ask the bus operator at the top any bus to Kepala Batas. Is air con bus just a bit expensive can reach kepala Batas Bus station. Thenfrom there you ask the ppl.

Good Luck