Based on standard adult pricing, no railcard discounts etc
Off-Peak Day Return is £32 if train goes through High Wycombe (I assume all Chiltern services into Wembley Stadium do?)
That's completely flexible on a weekend as all weekend classes as off-peak.
You technically select a train when booking, but this doesn't give you a reserved seat or confine you to that service.
If there are between 3 and 9 of you, you can purchase a GroupSave product, and you get a third off the equivalent number of adult tickets. Can only be bought from ticket offices.
BUT SOMETIMES THIS PRODUCT IS WITHDRAWN ON WEMBLEY EVENT DAYS SO CHECK BEFORE PURCHASE