I was nervous for the last game in Edmonton that the Eskimos would make the field goal, however I knew that a 50 yarder was unlikely. Here's a page with a chart on NFL success depending on the length. Here 40 yards is equivalent to 50 yards in the CFL, since in the NFL the posts are 10 yards behind the goal line.
From the chart, 40 yards out (equivalent to 50) is slightly less than 40%. Considering the quality of kicking is probably better in the NFL, the same chance for the CFL would be slightly worse. The chart is interesting too in that five more yards (55 yards in the CFL) and the chance halves to 20%. There's probably a difference between indoor and outdoor percentages so the Edmonton was probably even harder.
I would love to make a chart like that for the CFL, but it would be a lot of work. It would be interesting to flash it up on a TSN broadcast and I'm a little surprised they don't do it in the NFL (sometimes you see ranges, but that isn't a good way to measure it).