If you look at he last 20 years, in 11 years Non Premierhsip teams have made the FA Cup semi-finals. With the decreased importance of the FA Cup some teams (Premiership and Championship) may field weakend teams, increasing thr proabilities of larger margin Premiership wins but also Premiserjhip teams exits.
There is no reason the think that the likleihood of a non Premiership side raeching the SemiFinals is any less likely than over the past 20 years.. probably the reverse. I would therefore make this as 4/5 chance... I've had 8 pts at 5/2.