Are there even any solid release schedules for HBM3? I'll admit I haven't been paying much attention to it, but anything I've seen says 2019 at the earliest, with 2020 being much more likely. It might even be later than that if the past is any history as HBM has always fallen a little short of what the performance should have been and it's always been a pain to get the kinds of yields that make it economical outside of the top-end.
HBM has been a "lagging standard" from the beginning. That is, the memory manufacturer and the customer together figure out the specs and start making it, and then have JEDEC publish what already exists as a standard. It would be normal for the first time you hear about a new HBM version to be when a product using it is announced.
Not that there is any chance of HBM at $150. Not gonna happen.
If AMD really wanted to, couldn't they just use an IO die and some of the extra extra memory channels paired with some plain-Jane DDR4 in order to alleviate the memory bottleneck? It's probably not quite that simple, but the general approach seems easier.
DDR4 channels require pins in the socket. Two extra channels would require ~500 extra pins, when you count the extra ground pins the signal pins need for shielding. Socket AM4 only has 1331 pins total, it does not have an extra 500 pins free to use for extra memory channels. If you mean "stick DDR4 on the package", that won't work because there's no room. DDR4 chips only provide up to 16-bit wide interfaces, meaning you'd need 8 for two channels worth of DRAM. Can't exactly fit that on there.
I do personally think that a chiplet design greatly increases the chances we'll see HBM/other stacked DRAM in future designs. One of the main problems of making a really beefy APU that uses it is that it's going to be expensive, so it's only going to be a high-end part, so sales are relatively low, so it's hard to justify the mask costs. But, if you make your chip in two parts, one of which is made in fast process and the other in a process with low mask costs, suddenly the equation shifts a bit...
But that will not happen at $150. No how, no way. After paying for the chiplets, the DRAM, and the extra packaging costs, they'd be left with basically negative margin. Until HBM is really high-volume, which means it has been shipped in all the high-end stuff for a while, it won't show up in anything priced below $300.