If it's 10 hours of totally new levels then that's not that bad. Not great by any means but not all that bad, either. Personally, though, I wouldn't pay $60 for 10 hours but that's just me.
As for the rationale of being full price only because it has Guard included, that kind of rubs me the wrong way. Most people probably aren't buying this for Guard and would rather take the discount. I only see one SKU on Amazon, however, which is $60 and reads "BONUS game included" on the case, which would likely lead many people to assume that Guard is free. That's a bit shady.
What bothers me a bit more are the replies that seem to suggest that developers can get off lightly as long as you can play a game over a bunch of times. That doesn't make much sense to me. That shouldn't have all that much to do with the amount of content or the amount of work put in by the studio. I'm not going to give a developer credit for the hours I put in replaying the same stages or levels.