Hotwire is a great site, but personal experience says they offer better deals on domestic tickets - friends flew Miami - San Francisco (SFO) RT $245.
This last November when checking for flights from SFO to MAD (December 20 - 28th) we were being quoted over $1,000 RT each.
Hotwire can be great, but also keep in mind their partner
ORBITZ ...
You can pick your airline and time - we flew from San Francisco to Barajas (via New York) - on Delta - for $425 RT each.
That was a steal!