you can take West Rail line from Tuen Mun to Yuen Long Station and exit at G2 and take KMB bus No.64K to Tai Po Market Railway Station Bus Terminus
Fee:$5+$7.3=$12.3(for Octopus)
$5.5+$7.3=$12.8(for single ride tickets)
Times:13+66=79 mins(not including the waiting times and walking times)
KMB Bus No.64K Information:
http://www.kmb.hk/chinese.php?page=search&prog=route_no.php&route_no=64k
Hope above information can help you!