Hi there, I see.
Yep, the Nikon teleconverter for instance runs about $40 more than a generic video lens. I'm not sure how they compare optically however (dispersion, etc.). I know the Nikon teleconverter is made of ED glass (extra-low dispersion glass improves sharpness, color correction and minimizes color fringing). You have a nice camera. If it were me, I'd hold out and get the nice glass.

But, if you'd like to get a video lens, I'd check with the dealers about thread compatibility. I'd hate for you to strip your camera threads or your adapter threads. They would be able to help you with that concern.
Happy shopping,
Cheers,
Deb