Lowest I've seen this speed booster for Nikon to Sony Alpha E. Normally about £435 on Amazon. These are great if you use lots of legacy glass, effectively retains the original focal length, increases your max aperture by 1 stop and actually produces a sharper image. What's not to like? No AF I guess.

Obviously a bit niche so I expect cold votes.

Metabones site:
http://www.metabones.com/products/details/mb_spnfg-e-bm2

Review:
https://www.ephotozine.com/article/metabones-speed-booster-review-27042

Features:

Industry's best aperture ring covers a 8-stop range markings, clearly indicating actual number of f-stops. No more guesswork! (Patent pending).
Compatible with both Nikkor G lens & F lens (include non-AI & AI lens mounts).

1/ Advanced 5-element/4-group optical design incorporating ultra-high index tanatalum-based optical glass by Caldwell Photographic in the USA (patent pending).
2/ Increase maximum aperture by 1 stop.
3/ Increase MTF.
4/ Makes lens 0.71x wider.
5/The tripod foot is detachable and compatible with Arca Swiss, Markins, Photo Clam ball heads.

Note: This Speed Booster ULTRA is NOT fully compatible with full-frame Sony A7, A7R and A7S. When the Speed Booster® are used with Sony A7, A7R or A7S the camera is required to be used in the "APS-C Size Capture" mode.

Tips: Before mounting your lens to the adaptor, please make sure that the f-stop of aperture ring on the adaptor is turned to "7" (i.e. mounting your lens to the adaptor while only the aperture ring in the adaptor is full closed.).

Remark: This mount is manual aperture control. It does NOT support electronic aperture control.