I agree, it's really stupid that they can't quote you some of the fares without signing in. Depending on the route I guess, some routes do quote without signing in and some don't. If you can read Korean, there are some websites which you can use to list most available fares on KE, OZ, or other ex-Korea flights. You can also check expedia and even though they'll quote you in U.S. dollars, sometimes the price matches the Korean price but converted to U.S. dollars.