10x smaller doesn't mean 10x cheaper when things you're cutting corners on are warhead and rocket engine that is cheapest components per gram that also make the entire thing work. just to begin, 3M price per pop is the price that saudis paid for 650 SLAM-ER AShMs, along with training and limited tech support, and foreign sales are always fleecing customers like this. for domestic customer, price is half million apiece
Harpoon AShM warhead is something like 200kg, and for a very good reason. sinking ships is much harder than destroying land vehicles, and they want to sink something with 20kg warhead? that's a bit spicier than heavy ATGMs like Stugna-P. if that thing is enough to one-hit a ship then so is 127mm naval artillery shell (what they're going to war against, jetskis?). that immediately means they have to compete with a naval gun at ranges less than 40km, and they will lose. also such small ship probably isn't that much of a threat at distance
with anything big enough to get notice of airforce there's quicksink, that is JDAM kit modified to dive under ship in question and break its keel. with one-ton warhead it costs something like 120k, and it's already tested and will be ready much faster than whatever they're cooking. they're simply noticed that chinese navy is stocking up anti-ship missiles and want to get on the hype train. or maybe they're copying ukrainians? neptune has 150kg warhead and various drone boats carry even bigger load (200-850kg and maybe more than that)