Last estimate I saw was a mission cost of $500 million, so an 18-month delay at a discount rate of 5% would be around 35 million. I don't know enough about the launch vehicle to say whether a sufficiently-heavier rocket would cost more than an additional $35 million, but I certainly wouldn't be surprised if that's the case.

The actual comparison, btw, would be to delay spending two years and launch in 2013. That gives you the ability to compare costs.

Some launch windows are better than others, too, so that might play a role.

Final note: given the ESA politics, this might be their backup plan - blow their 2011 launch deadline, get some more money, and launch in 2013 while not missing the "official" arrival date. Just like the Deep Impact mission.