Have you checked out the Sofitel (or have they just changed it to Pullman?) ? They're VERY competitive...basic rooms under $100, exec rooms under $150 (inclusive of taxes, breakfast and, for DLX or EXEC, internet), I really liked the service, and found the location better than Hyatt (which is not that far from the centre, but sits atop a hill with not much very near it).

However, IMHO all hotels in SEL are eclipsed by the Fraser Suites. Alright, they're classified as "serviced apartments", but IIRC they'll let you stay even for one night. VERY spacious, located slap bang in the middle of Insadong, and cheaper than an executive room in a 5-star hotel.

I didn't get to stay there, but I was walking by and got a manager to show me the inside, and it's an order of magnitude better than what any hotel can offer you...