If you purchase an open return train ticket to Southampton, the cost is £84.00.
If you split your ticket the same journey on the same train is £50.50 as follows:
Birmingham New Street to Banbury - £19.10
Banbury to Reading - £12.40
Reading to Southampton - £19.00
Total Price £50.50 no change of trains
If you start your journey at Birmingham Moor Street and change trains at Banbury, the cost is £45.50 as follows:
Birmingham Moor Street to Banbury - £14.10
Banbury to Reading - £12.40
Reading to Southampton - £19.00
Total Price £45.50 CHANGE TRAINS AT BANBURY
Hope this helps.