if just from Osaka to Tokyo then back again the price will almost be the same.  if u plan to go other places (ie Kyoto etc) then it's worth it.  seriously, 7 days for these 2 places (Osaka and Tokyo) is really not enough.  I advice u to just stick to Osaka / Kansai area, don't waste time travel, my trip was 10 days and it's barely enough to cover roughly 50% of my planned schedule for these 2 places.  u have to understand, if u are not familiar with japan u will need a bit more time to find your way around in a free and easy trip.

I like Osaka more than Tokyo btw, it's more relaxing.  the only reason i would go to Tokyo is just because of Tsujiki Market.  I'm not a fan of shopping, so Tokyo has nothing to offer me except the seafood.  oh, and THE BEST MEAL i've had (except for the sushi in Tsujiki) is in Osaka outskirts (Minamino area) and it cost only 600 yen, super local food and local style, full meal with unlimited supply of rice, eat until u burst.

I agree on the first part. 7 Days just stick to one region.