It is more difficult to make one for twice as large 135 sensor image circle than m43 IC.
With lenses like this of such small angular field of view (about 8º diagonal for 300mm in 35mm format) there is no significant extra challenge in getting an image circle covering the 36x24mm frame. Optical designs for lenses like this naturally produce an image circle far larger than needed, with diameter not much smaller than the focal length: this is then cropped by the sensor to a small central portion of the image, and probably also cropped before that by the lens body and lens mount. Note how little the MTF of long lenses falls of from centre to edge compared to normal or wide-angle lenses.
Two more important differences between a 300/4 lenses for Micro Four Thirds vs Nikon F-mount are:
a) Resolution needs in lp/mm, which are probably set higher for the smaller format and the smaller photosite spacing typically used there.
b) Sales volume and related economies of scale: Nikon F-mount clearly has higher sales volume, and 300mm gives a more widely used FOV on either Nikon FX or DX bodies than the same 300mm does on a MFT body, so I am sure that the Nikon 300/4 will continue to significantly outsell the Olympus.
I would guess that the latter is the main reason for the price difference. The most economical way to get extremely narrow FOV options like 300mm for 4/3" is to use adaptor-mounted lenses for larger formats, but of course that loses some conveniences in AF and such.
