Im pretty sure the M18 was issued to the USMC. The surplus M17’s are bringing the money. $1400 was cheap.The military trial pistols were Sig M17. The m18 is a compact version of the m17 and as far as I know was never issued to any military unit.
I had one of the military issued m17 snd sold mine for $1400 a year ago. You are very mistaken about the values.
https://www.militarytimes.com/off-d...us-marine-corps-adopts-m18-sig-sauer-handgun/