Anybody that has a 1-6 travelcard book an East Croydon to Brighton return and you can go for £6(look on lowest fare finder), or from London Bridge it is £17(LFF again).
These prices are based on people having a Network Card to ensure the discounted price.
Brighton will be doing the same deal on the match ticket with you being able to use the trains for free from Haywards Heath to Falmer station.