There is no special discount for Coldplay visitors. The ticket for the concert includes free public transport in Zurich only (ZVV). You will need a regular ticket to get from Basel SBB to Zurich. The return ticket is CHF 68.00 full fare.
If you take the IR36 train, you need a ticket from Basel SBB to Dietikon. From Dietikon, your concert ticket is valid. Get off the train in Zürich Altstetten to reach Letzigrund stadium. Note: The proposed ticket combination is only valid on IR36 trains; any other direct train needs a Basel SBB to Zurich HB ticket.
More information on the Coldplay concert and using SBB for it, you can find here: Coldplay – special event trains to the concert | SBB
There are no extra trains planned from Basel, whatsoever.