It's not really a simple answer.

The SSME (aka RS-25) is a complex, high performance engine that took a lot of effort to be actually reuseable in spite of itself. Most rocket engines >are< reusable, they're just not recovered; SSME really pushed the limits of technology.

The RS-68 was designed from the beginning to be just high thrust and moderate Isp while being less expensive through a simpler design that doesn't have to operate at exceptional pressure and temperature. Unfortunately, the lower unit cost of the RS-68 seems to be compromised by its lower Isp and heavier weight--the Ares V has had to be redesigned larger and larger to keep the desired payload to the point that it's become impractical.

An uprated regeneratively-cooled RS-68 would help matters, but it'd be a major development program--as would just about anything other than a stock SSME or RS-68 would be.

A RS-25E is being proposed but I'm not sure how much of SSME would actually carry over. Trying Googling for COBRA, an engine development program that would incorporate new manufacturing technologies and a simpler powerhead design with existing SSME turbopumps. If the upper end of thrust at 1 million pounds could be reached, this could allow for a smaller and less impractical Ares V. I kind of like COBRA, but it's still not cheap or quick to develop.

Consider the Russian RD-0120. It's an example of an expendable SSME-like engine that flew on the Energia. I'm such they'd love to quote prices on whatever quantity you'd care to order, as Lockheed-Martin did with the RD-180 ($1 billion for 101 engines, not a bad deal, eh?). It might be a great choice, and it's essentially off-the-shelf. Politically...even I'm not much in favor of it.

A significantly higher-thrust SSME-like engine would benefit Direct just as much as Ares V, and Direct is a more sustainable architecture. But alas, we're trying to do an inherently expensive program on the cheap in the middle of a major economic downtown, and adequately funding a real program seems unlikely in these modern times. (Ah, for the glory days of Apollo, when Federal money flowed like water...I was in high school then.)