According to recently rumors on the web, a new Sony FE 20mm f/2.8 lens is rumored to be announced next. Sony already has A-mount version of this lens, costs $748.00 in US. So the rumored FE mount version of this lens will have same price range.
The Sony FE 20mm f/2.8 lens is one of the rumored upcoming FE lenses from Sony, other rumored FE lenses are FE 135mm GM lens and FE 400mm GM lens. All these lens are to be announced in late 2017 to early 2018 along with Sony a7III.