Forget the GO. Its for commuters, and takes longer than you'd expect.
Find a way to Barrie Bus Terminal which is downtown Barrie. From there you will frequent sheduled South bound departures by Greyhound $15 to $18 I think? The Ontario Northlander Buses also run through a few times a day, and they all go to Yorkdale, and Bay and Dundas stations.
www.greyhound.ca
www.webusit.com
As for Borden to Barrie I am unaware of a bus. Cabs will be high dough-lah...So split it??
I used to commute to Toronto from there and these buses were the quickest way in.
EDIT
I still posted this despite Ludoc's post just incase, I forgot if they went to Union...The other two for sure
And fyi Bay and Dundas is main terminal for buses from Toronto. Union for trains... Incase anyone reading is new. I made that mistake - Long Christmas Eve