When HK first came out with their version of a piston-driven AR prior to 2010, it actually was very much better designed and executed than almost all the others out there. So back THEN, it might have made sense to spend the extra coin. But in the 15 or so years since then, everybody else has been learning from HK (and others), and various small details and improvements have been adopted by many of the models available from their competitors.
Back then, I truly was all hot and bothered to get my hands on an HK416 upper for myself,...if I could have afforded it,...and found one. (I did find some HK416 mags, though, and they have been great.) But having built many piston ARs from different kits and parts sources, I've now seen how there isn't really that much difference anymore. The SIG rifles are just as well engineered and fabricated now from what I can tell. I still think $2500 for a SIG is too much, but I'd more readily spend that amount than $4000 for an HK.