There is really no good reason why the same battery (produced in China I'm sure) should sell for $109 in Canada and $49 in the US. I just checked on line at Bestbuy.ca vs Bestbuy.com and that is the price difference. I did notice that Bestbuy.ca is offering their house brand Digipower battery for the D5100 at $49.00. I might be tempted to try one of those.
I did luck out one day and found an "open box" sale at Best Buy for an accessory kit for the D5100 that included a battery, camera case and other goodies, but because the box was damaged it was on clearance. The total package was 50% the cost of the battery. I didn't want or need the case or the other goodies, but got the battery I needed for about $60 instead of $110, and the goodies were just gravy.
I also was tempted by the real cheap ones on eBay, but after finding lots of people complaining they did not work I decided not to waste my money. I did read that while the cheapo's may not work directly in the camera, if you buy the cheapo battery pack that takes two batteries, and use a real Nikon on one side you can use a cheapo on the other. The battery pack and a pair of cheapo's was about $40 if I remember correctly, ... that is still a little tempting. Anyone here prepared to admit they use that set up?
BTW this price difference between the US & Canada is fueling a multi-billion dollar cross boarder shopping spree. One thing I did learn shopping in the USA, some retailers no longer accept Canadian Credit Cards, (Visa or MC or whatever) because you MUST have a ZIP code. I even found gas stations in the mid-west that would not take my card. They pointed to their over priced cash machine and said withdraw the cash (for a fee) and then buy your gas with the cash. I said that was not very friendly to Canadian visitors, they said they don't get many Canadians ... no wonder. I learned quickly to stick with Exxon and Shell who did take my card. Even shopping on line can be a problem without a validated ZIP code, retailers like Newegg.com won't take my Visa. (Bestbuy.com by the way does allow me to use my card).
So, note to self, next time I'm in the good ol' US of A, pick up another Nikon battery at less than half price.