I think some of the altcoin mining makes use of large amounts of RAM, so you've got some of the miners buying up a lot of RAM and driving prices up as a result.

Usually a combination of demand and supply. With hardware there are huge periods of overproduction and even ruinous competition and periods of scarcity. Prices will come down to long term average again.

I've been thinking for a couple of years already of pushing the RAM of my old gaming laptop from 8GB to 16GB, but I guess I will still postpone those plans...

On a related note, are HDD prices still up due to floods in Thailand hard-drive factories? :)

Put off an upgrade for my second system and got a PS4 instead last friday. Just not worth it with the current ram costs

Honestly, DDR4 isn't worth it.
Just get some DDR3 RAM, you won't notice any difference in speed and it's way less expensive.

Edit: Make sure your motherboard supports whatever RAM option you chose.

Shouldn't you actually go with the memory your motherboard uses instead?

If your motherboard supports DDR3 go with DDR3 because it gives you more bang for your buck.
But if your motherboard only supports DDR4 then I guess you are forced into that.

Edit: Make sure your motherboard supports whatever RAM option you chose.

So you're saying that if you want to upgrade your current machine, you don't have a memory choice, and if you want to buy a new machine, go with the older technology because it's cheaper instead of attempting to future proof your machine, right?

"So you're saying that if you want to upgrade your current machine, you don't have a memory choice"
This completely depends on your motherboard, look your motherboard specs up online.

"go with the older technology because it's cheaper instead of attempting to future proof your machine"
Well I'm personally not a big fan of future proofing but if you want to future proof your machine just get a motherboard that is compatible with DDR3 and DDR4 RAM.

Edit: There are no motherboards compatible with DDR3 and DDR4, there are motherboards compatible with DDR3L and DDR4 though

I don't think those motherboards exist, since DDR3 and DDR4 use different connectors. Can you link me one, just to satisfy my curiosity?

Oh my bad, you are right, there are no motherboards that support both DDR3 and DDR4.
Although there is support for DDR3L and DDR4.