Assuming they get the same amount of spectators that would've went to Upton Park, there is no difference in gate receipts as there is no overhead on a per-seat basis.
It's going to cost them considerably more to run the Olympic Stadium than it is to run Upton Park. Therefore they're going to have to generate at least that extra amount more from gate receipts/matchday income at the Olympic Stadium to make it worth their while moving.
Whether they can generate that extra income will depend on their fans reaction to the running track.