A cab is certainly a good option. I would agree that $5, or maybe $10 would cover it. After all, the Poly is really only probably .25 mile away from WL as a crow flies, but it's too dangerous a walk with a fairly busy street to cross, and then the super-busy TTC.
Other than a cab, the quickest way to the Poly from WL would be the WL bus to MK (direct), then the Poly boat to Poly. The reason I prefer this to the monorail is that the monorail will have at least one or two very busy stops to make before getting to the Poly, while the boat might just make a direct run for you or will stop at GF at the worst, if you're a little lucky.
From the Poly to MK, I'd monorail over, since you have only the GF to stop at and it's one of the faster stops.
From MK to WL at night, I've only been able to take the boat, as I've not stayed there since they instituted the direct bus route from MK to WL, so I don't know which would be faster at the end of the day (keeping mind the crowds at the bus stops).