You can get an Off Peak Return (SVR) from Waterloo to Neath via Salisbury and Cardiff for £50.70 (£39.35 + £11.35).
On the SWT site, click the "other routes" button on the price page (which gives you fares via Paddington), and then click trains via Warminster/Salisbury.
It doesn't seem to want you to start at Clapham Jn.
Double the journey time though.