I often go to the gardens, and don't mind company on my visits :-)

I would recommend you first get yourself an Acces Montreal card. This allows you free entrance into the gardens (but not the greenhouses, for that you have to pay or you have to join the 'Friends of the Garden'), and it gives you a discount for many other Montreal events, including the International Flora Show. The cost for the card is $7, which is less than the cost of 1 admission to the gardens (actually it's a bit more than $7 for the card, because you have to have a photo, but I believe one of those instant photo booth pictures are acceptable)

BP