From January 2020, two trains will leave Exeter St Davids for London within minutes of each other. A seat on the 0645 to Paddington will cost you £279.60 anytime return while a seat on the 0641 to Waterloo will cost you £80.80 off peak return. So you save 71% on the fare to Waterloo but the down side is that the journey takes 84 minutes longer.
If you need to be in London before 0900 you might like to consider the 0510 to Waterloo at £139.60 arriving at 0846. Compare this to the 0547 to Paddington getting in at 0836 and costing £279.60. The slower trip saves you 50%. I suppose it depends which bit of London you need to get to, who's paying and how early you're prepared to get up!!
These are buy on the day fares. Advanced purchase fares are cheaper.