I'm not sure about bluray, but for 4.7 GB DVDs, it's still possible to get packs of 50 for $30 here pretty often, sometimes lower. That makes 235 GB of storage for $30, but it's not as convenient to store stuff on them as it is with SD cards.

I wonder if the lifespan of a SD card is better than a DVD-RW?

I think my friend has a 1 TB microSD o.O

100,000 cycles is accurate. That's the most common estimation.

Solid-state disks usually employ write-wearing to make sure data is evenly distributed across the disk, and prolong its overall lifespan. I'm not sure if this is necessarily true of memory cards and FlashDrives, but it could be. That said, I do think Flash memory is really stable as compared to some other storage. HDs with moving parts are much more likely to fail. I think this is especially true USB hard drives.

I'm not sure how many times optical mediums can be rewritten. I imagine much less, since they're mostly intended for "write once, ready many" storage. I never bother to rewrite discs, or add new data at a later time. I just burn it all at once and close the session. I'm too worried adding or editing data will corrupt some sectors and cause a cyclic data redundancy error upon trying to read it again. Then again, I'm paranoid when it comes to backing-up my data.