If you can deal with returning on Saturday, I managed to get an itenary @ 40.70 (+£2 ish)
Thursday 19th September.
09:17 Torquay - 09:28 Newton Abbot
09:34 Newton Abbot - 09:53 Exeter St David's
10:07 Exeter St David's - 10:19 Exeter Sidwell Street (local bus)
10:35 Exeter Sidwell Street - 20:00 Preston (Lancs) (National Express)
(Note, you may want to take an earlier train or a taxi through Exeter, to make this connection more comfortable, however, the train before the 09:17 is the 07:17)
Saturday 21st September
09:45 Preston (Lancs) - 17:45 Exeter Sidwell Street (National Express)
(Walk)
18:27 Exeter St David's - 19:12 Torquay
Fares:
£27.60 (Inc booking fee), National express
£13.10 Torquay to Exeter St David's off peak return
~£2 for the bus.
I think this may well be the best option; spend the night in a coffee shop, and have a sleep on the bus.
Edit: to get the NEx fare, I typed Exeter -> Preston