For reference, given the proximity of Birmingham International to the convention venue it's no cheaper to buy a ticket in advance than it is to turn up on the day and buy it from the Ticket Office (which is open 05:40 - 21:00 Monday to Saturday and 07:40 - 09:00 on Sunday.
If travelling by train, the best option from Birmingham International is to go to Nuneaton, which involves one change of train at Coventry. If you leave after 09:00 you can get an Off-Peak Return (valid for return within one calendar month, travelling after 09:00 Monday to Friday in both directions) for £9.60. If you leave before 09:00 you can only get a Single each way, which costs £7.70 per journey.
Penalty Fares do NOT apply on CrossCountry or Virgin Trains services, but if you board a train without a valid ticket and the Ticket Office is open and/or Ticket Machines are working, you will only be sold a full price Single on board.
Cheers,
Southie, your friendly neighbourhood railtig