>>Then they made the Nikon SP...Leica ended up making a new lens mount to keep up<<
The M lens mount came out in 1953/54, which was three to four years before the SP (1957). The M3 was Leica's response to the top quality cameras coming out of Japan (Zeiss's response to Japan was lawsuits instead of innovation, but to be fair, Zeiss had massive corporate and legal problems related to postwar breakup of Germany -- preservng and salvaging as much of the company as possible was their priority -- but it showed in the lack of development for the Contax line).
The SP was a counter-response to the M3. The "3" in M3 allowed you to shoot three different lenses without a separate finder -- 50, 90, 135. The SP upped this to six-built-in frames ... 28, 35, 50, 85, 105, 135. Leica achieved six framlines about 20 years later.
Quality-wise, the SP and S3 cameras are Nikon Fs with a different viewing mechanism.
The Nikon designs include an extremely dependable Leica-based shutter and a Leica-based rangefinder mechanism (but the central RF patch on a Nikon is not as visible and defined as on a Leica M camera). Unlike Leica, the Nikon RFs, beginning with the S2, have 1:1 lifesize viewfinders.
The recent lower prices for Nikon S3s and SPs make them again affordable for users, as opposed to collectors. I think Nikon collector prices have passed through their "tech bubble" phase -- they have ceased being crazy speculative and now more accurately reflect their inherent quality.
Pricewise, from the very beginning, Nikons were never intended to be cheap cameras and lenses. In the 1950s, Nikon was comparable to buying a Lexus instead of a Mercedes.
I have three original Nikon RFs that have all seen daily use. My S3-2000 has been my grab and go camera for the past year.