The original Sig Pros were originally chambered in 40 S&W and then .357 Sig. They were the 2340. Early 2340s were made in Switzerland and the slide are marked accordingly. Later production was moved to Germany. They were then introduced in 9mm as the 2009. They did not sell well in the US until they were discounted and liquidated at CDNN and then they moved quickly at about $300-$350 IIRC. They were adopted by the Swiss IIRC.
You have to take into consideration that these were the hay days of the classic Sig LEO trade ins and CPOs. When you could get a LNIB P228 CPO for $500. So a $500 polymer Sig with a propriety rail was not going to sell well. Even then the Sig Pros were well respected for their trigger system which is different than the standard P series Sig.
The French police put out an RFP for a new pistol. Sig modified the Sig Pro 2009 and submitted the pistol. There were more removeable back straps, standard pic rail, night sights and the Iffalon finish. The French approved the Sig bid and they planned to use the gun for 20 years. They did this in 2000 and therefore named it the SP2022. It was not named this before Sig submitted it. As pointed out the economy of scale gained from the French contract allowed Sig to produce the gun at a low cost and still make a profit.
The gun gained popularity when the triple serial numbered German proofed guns were being sold with night sights and 3 mags for $400. The production was then moved to the US and changes were made to the extractor and other parts were replaced but they continued to sell in the sub $500 range. They still pop up now and again in the $400 range.
For the money they are good guns. The one thing you have to consider closely is that SP2022 trigger is different than the Classic P series triggers. It is smooth and clean but longer than the P series guns like the P226. People compare them all the time to the P series guns but exept for the name Sig on the slide and the lower cost of entry they are not very Sig like. Does not make them bad just makes them what they are. If you can try before you buy. Don't over pay. If you can get a triple serial number German proof gun they are the best of the breed IMHO and command a bit more in the used market. They have a serial number on the slide, underside of the frame & barrel.
I cannot stress enough that comparing it to a classic P series like the P226 is not a fair comparison. The P220, P226 and P228 are the guns that build Sigs stellar rep and while the SP2022 is a good gun it is not IMHO in the same class. Also people bringing up the P320 as an option does not make sense to me. Again besides the Sig logo on the slide, 9mm chambering and the fact it is a polymer are they similarities between the 2 guns. Look at the OPs post the P320 does not meet the criteria. Just because it has a Sig logo does not make these guns similar to each other in feel and function.
I like mine and it serves as a bedside gun.