I can get the fare from Brighton to Redruth one way on 19 June down to £25 single (albeit on slow trains via Bristol) and on 26 June to £28.50 (via Bristol and London). Not sure you are going to do better than £53.50 return (or £49.80 in the last bit of this message).
19 June - 0900 - 1812
Brighton to Salisbury £4.40 single (0900 from Brighton)
Salisbury to Bristol Temple Meads £5.45 single (1113 from Salisbury (same train))
Bristol Temple Meads to Redruth £15.15 single (1356 from Bristol Temple Meads)
Brighton 0900
Bristol Temple Meads 1230
Bristol Temple Meads 1356
Redruth 1812
26 June - 0843 - 1657 (also possible 1143 - 2027, 1516 - 2327, 1616 - 0056)
Redruth to Bristol Temple Meads £15.15 single (0843 from Redruth) (1316)
Bristol Temple Meads to Brighton £13.35 single (1330 or 1400 from Bristol Temple Meads)
Redruth 0843
Taunton 1142
Taunton 1218
Bristol Temple Meads 1316
Bristol Temple Meads 1330
London Paddington 1507
Underground
London Victoria 1559
Brighton 1657
Actually, looking later in the day, it is also possible to do (1820 - 0228) for £24.80, so that is under £50 return.
Redruth to Bristol Temple Meads £8.15 (1820 from Redruth)
Bristol Temple Meads to Brighton £16.65 (2200 from Bristol Temple Meads (same train))
Redruth 1820
London Paddington 2341
Underground
Blackfriars 0102
Brighton 0228