A couple of afternoons this week, I've travelled down to Trowbridge and back, from Nailsea & Backwell.
There is a choice of tickets:
Anytime Day Return (any permitted) = ^11.20
Off-peak Day return (via Bath Spa) = ^11.60
There are also Advance tickets available for ^10.50 or ^11.50 single (also some cheaper). A single ticket is just ^10.00, so both of these advance tickets are in fact more expensive.
I expect that that ticket would be valid via Taunton Chris...?! But I don't know for definate...
You could go "Nailsea & Backwell > Taunton > Castle Cary / Westbury > Trowbridge..." But why you'd go that way is beyond me...

Nailsea is in both Bristol and Taunton Group, so it may well be permitted to travel via Taunton (could someone confirm?). In fact, the ^11.60 pointed out by Chris is cheaper than Nailsea to Taunton (SDR ^15.70), so this could be another one of the crazy breaking journey anomalies to add to the wall of fame.
Edit: Scrap my last comment. I've just found out that it must be the shortest distance of the two routing points, so a journey via. Taunton is not permitted. This then makes the "Bath Spa" ticket useless, as there is no other permitted way of completing the journey.