Everything Novoflex makes is expensive. In general, I'd suggest considering autofocus stacking before bellows, tilt-shift lenses, or reverse adapters. It's an inexact replacement for movements and trades less hardware for compute time in "post" processing but, overall, stacking is probably more versatile and a bit easier for most things. I wouldn't suggest it as a sole tool for insect macro, though.
It's concerning Novoflex doesn't specify the thickness of the MFT-RETRO. It appears likely to consume a substantial portion of the working distance available at magnification and it's my experience the ~62 mm outer diameter needed for mounting a 58 mm filter is easily problematic for subject lighting when working distances of ~30 mm are unobstructed.