Online Travel Booking and Bidding Agencies - same day round trip expensive?

I try to book a same day round trip from San Francisco to Portland without overnight stay. It is more than $900. But if book a round trip with different start and return dates, the price is only $240.

Why is that?

Because carriers penalise business passengers for short & midweek stays that don't include a Saturday night stay. This is normal practise but as you noted it is cheaper to buy two seperate tickets and not use the return legs.

Although there is one point to keep in mind some carriers are getting wise to this and in their terms and conditions they may state that if you don't fly on all sectors purchased they can charge you the published fare for the oneway flown!