United is offering San Francisco to Denver for $48.30 each way from next week through first week of August. There are fewer dates departing SFO with Mondays being most available and many more dates for departing Denver. This is Basic Economy so only carry-on bag for free. Add $9 to $25 to self-select an economy seat each way and $25 for a checked bag.
One benefit for having achieved Star Alliance Gold through Aegean Airlines this month is I get a free checked bag on United Basic Economy flights.
United Airlines San Francisco SFO – Denver DEN – San Francisco SFO