Very little really but that's not the point, they cheated. Yes, the Lib-Dems and Remain got done too, but just a fraction of the amount for a technical infringement for Remain.
The real issue here is that the Electoral Commission wanted to drop the case against Leave EU and were forced to keep it open. They then did nothing until a whistle blower blew the whole thing open.
http://www.bbc.co.uk/news/uk-politics-44080096