kinguin.net seems to be the cheapest place for SH5 if you are not too bothered about having a Retail DVD Disc , Site seems legit uses Paypal price £2.67 UK ( $.3.52 US) for UPLAY Key .
The Ubisoft server seems much more reliable than it was a few years back so its not a bad option to download it from them
The Prices for the "GOLD" Edition are in some cases double price which is totally pointless. I got the Standard Edition after I downloaded the patches to v 1.2 it changed to "Gold Edition" at no cost
The Steam version is very expensive and as you say no good for modding
I can't 100% guarantee that all sites offering CD Keys are Legit but I have used a few got huge discounts with no problems, I think its something to do with non EU KEYS ,not sure
Best Regards
Simon
WOLVES OF STEEL MEGAMOD