We might have this issue next year. The fact that USC only has RD makes deciding whether to apply ED elsewhere pretty tough. My D23’s current favorite school, RIT, is a good match; we are sure she’d get in via ED and think she’d still likely be accepted via RD, but what if we’re wrong and she doesn’t get accepted to RIT because she applies RD just so she could also apply to USC, which is a longshot at best? In her case, an applicaiton to USC kind of makes sense: It is the top-ranked school for the major she wants. Still, it would make no sense to apply RD to USC if we know she’ll get in ED elsewhere, and is it worth even a small risk of being rejected by RIT, which has as a very good program in her major, just to have a longshot at the school that has the ‘best’ program?
In your case, however, it doesn’t sound like you’re aiming for USC because it has the best program for your major. If you can get in somewhere else that has a better program and happens to be cheaper, too, that’s probably where you should go. There’s no rule anywhere that says you can’t be a Trojans fan unless you went to school there, and maybe there’s still hope: Could you go there for a postgraduate degree?