Are they worth it? YES
Are they expensive? For lower lvls YES for 10+ YES but not as bad
What do they do? Each Entrance gives you 1/2 star for every match.
1 entrance = 20k fants
20 entrance = 1 xp
1 xp = 400k fants
These help feds and Popularity gains(fans). They are very important for feds, and can give you increased AT's over time. Since At's are now like s1 many ppl have sold SA's for Entrances.
NOTE: You shouldnt buy entrances if you dont have the Fants for stats.