That Sulwhasoo Overnight Vitalizing Mask is awesome and one of their more affordable items for sure. I've purchased that travel sized tube before myself, and a little goes a long way. You can also get large sample packets of it on eBay for around $10 for 10 packets. Each packet is 5 mL, so you can get several uses out of each packet and it ends up being about $10 for 50 mL of product! I usually buy those from topinkgirl because she ships fast and has a great rating. Link to her listing for the Overnight Mask here

I've also bought the Sulwhasoo sample 4-item kit for less than $12 on Amazon (it's even Prime!), and found that to be a good way to try the items AND have the cute little Sulwhasoo containers to decant samples into later if I want to, though these are my favorite pretty decant jars and bottles.

EDIT: Also, thank you for linking that Reddit review on the Secret Key cream! I hadn't seen that!