looking to extend the power cords on my Minn Kota Powerdrive to reach the batteries in the back of the boat. How can i do this in a cost efficient way?

2 each; 6 gage wire butt splice, 6 gage wire long enough for your extension, 6 gage terminal lugs with 3/8" hole to fit your battery connection. Crimp tool.

Tom

Get one length of wire in red, and the other in black.  Depending on the length, you may be able to find a set of jumper cables in six gauge copper cheaper than individual wires.  They will be in red and black, and, they will be joined together for most of their length.

Keep in mind, many jumper cables are aluminum.  Be sure to get them in copper. 

I know you are trying to save money, but be careful!  Don't use wire that is not heavy enough and make sure any splices in the wire are extremely tight. There is a lot of current going through your trolling motor wires. Any loose connections will get very hot. The insulation on the wires can melt and start a fire. Saving a couple of dollars on your wiring could cost you your boat or even your life. For safety sake, do it right!