I've been watching prices on Canon 1.4x teleconverters for a while. Looks like the going rate, new or used, is about $280 US.
I've been wanting to extend the range a little on my 70-200mm f/4L for use in outdoor sports events, and figured that would be the way to go.
I've also lately noticed a few Canon 100-300mm f/5.6L lenses for sale on both Ebay and FM forums, for not much more (generally in the low $300's US).
Considering there isn't much price differential, which would you go with? Throwing the Tcon on the 70-200 would make it (for all practical purposes) essentially almost the same as the 100-300mm f/5.6. (Yeah, I know, it would actually be a 98-280 f/5.6L).
Can't afford, and don't really need, 2.8 glass right now, since I primarily use the lens for outdoor events where fast shutter speeds are available even at ISO-100 most days. (Though I realize the DOF advantage of a lower f/stop lens).