Was reading on the National Express site that, yes, its £4 but you can use it on all NX buses.
From their website:
"City's next home game is the npower League 1 match against Oldham on Saturday 19th January 2013.
The kick off time is 15:00 at the Ricoh Arena
Service 101 operates from Pool Meadow bus station directly to the stadium, with the first bus departing at 12:30.
The COVENTRY MATCHDAYSAVER is available on any National Express Coventry bus from 4 hours before the kick off and allows you to travel on any National Express bus. It is the only ticket that allows you to travel on service 101
We have introduced a CHILD COVENTRY MATCHDAYSAVER for children up to 15 years of age. It costs £2.80
The price of the Adult COVENTRY MATCHDAYSAVER remains at £4.00."
4 quid for the return only seems very steep. Probably a new driver got confused ??!!