It doesn't quiet work like that, some do a monthly sub for a "sports package" for about a tenner, so it's active all week, some do a weekend pass.
The cheapest way to do it is to look at the games which aren't shown on your sat or cable box.
If you like Liverpool there won't be that many, you can get some 3pm's on the old Setanta channel which I think is now re named as SSP 5
Yes, I seen the Burnley game it was dire lol
You can buy a cheap box such as an MXQ if you just want it occasionally and just pay a server for the games you can't get on your sat or cable box.
If you intend to use it a lot go for a Mag box like Running out mentioned.
Bought mine for this season so i'm still learning.