I ventured out to the Toys 'R Us in Seoul to check out the selection and do a little bit of price comparison. They had a lot of the same merchandise with some additional brands that I didn't recognize.
Melissa and Doug Toys, like this sorting clock were about the same price. Match Box cars were about the same price as well.
Other things are more, but worth buying to avoid shipping costs and waiting for delivery like:Baby Bjorn Carrier Air is $99.99 in the States and 128,00 won at TRU in Seoul. (The Bjorn is not my favorite carrier, just using it as an example. I think the Ergo and Beco are much better).
Other things were significantly more like the Boon Potty Bench which is $29.99 and was 48,000 won.
Infant Tummy Time Mats were all over 100,000 won which is a lot! Children's bicycles were all over 100,000 won, most over 150,000 won.
So, it is definitely worth the trip, but depending on what you are looking for and how soon you need it, it might be better to order online.
To get there: Take Line 1 to the Guil Station. TRU is in the basement of Lotte Mart. There is a big red sign for Lotte Mart. You can see it from the station.