United Airlines has Chicago to San Francisco route on sale for as low as $63.10 each way, however there are only a few dates in late April at $63, more dates at $69 and a variety of dates from $89 to $109 each way for travel from late March to May 2016.
Easiest way to match low low fare dates is search one way fares on Google Flights. The low United fares are a response to Frontier Airlines more frequent dates throughout April with $63 one-way fares.
If you want to travel between SFO and ORD for six or seven days in April, then United can match Frontier low fares. But the red-eye low fare flights kind of suck. Arriving in SFO at 2am is a waste of a hotel night for a traveler.
United Airlines Chicago ORD – San Francisco SFO (nonstop flights)
$126.20 round trip Tuesday April 19 – Monday April 25, 2016
United miles earning base fare = $98.00.