If you want 3x and 65 degrees apparent, this is easily done without prisms, but with an image rotated 180 degrees. Goes deeper, but not as fun being upside down. Then there is the Amici prism that aims at the zenith easily, but they cost $40 each for coated ones (Agena sells uncoated), and the clear aperture is only 18-19mm. You could buy an Abbe-Konig prism, but they are expensive, likely small, and hard to mount.
Alas, the Nikon 2x may be the best we can do for now.
Oh, you can also mount a relay lens and use an Ethos eyepiece, for an upright image and ultrawide view. But then you are talking big.
The Galilean system has less field curvature than the Keplerian.
If $30 per teleconverter is expensive, look at the cost of an eyepiece for a Keplerian. This is still the cheapest way, it seems.
Edited by stargazer193857, 03 October 2016 - 07:12 PM.