Good read:
Kirk Cousins' trade value and the NFL-wide impact of the drama in D.C. - 2017 NFL offseason
A swap of first-rounders makes more sense, with Washington exchanging the 17th pick for the second selection. Chase Stuart's draft value chart measures that swap as worth 13.6 points of draft capital, roughly equivalent to the 27th selection in a typical draft. The 49ers have two fourth-round picks after picking up a compensatory selection for Alex Boone; maybe they send the 109th selection to Washington and get the fifth-round pick Washington grabbed from the Saints (154) in return. Now we're up to 15.8 points of draft capital, roughly equivalent to the 19th pick.
The 49ers could finish out the deal by including Eric Reid, who has been an excellent safety at times while struggling with various injuries. Washington has been a mess at safety over the past several seasons, and Reid -- in the fifth-year option of his rookie deal -- would be a massive upgrade on DeAngelo Hall and Donte Whitner in the defensive backfield. This wouldn't be fair market value for Cousins, but picking up Reid and what amounts to a mid-first round pick for a player who doesn't want to be in town and is likely to leave after the season would make sense.