If you want to take the direct train each way, I don't think there's anything much cheaper than £45, splitting at Stafford.
With your outward journey being early in the morning, there's no realistic itinerary that, for example, takes you along the cheaper Trent Valley route changing at Crewe and Nuneaton.
You could save just over £10 by taking the Northern stopper between Stockport and Stoke - their Advance tickets are very cheap on that service. You'd then buy Stockport-Stoke (Advance, from £3.20), Stoke-Stafford (£7.70), Stafford-Coventry (£19.80), and another Stoke-Stockport Advance on the way back. Total from £33.90 (may be more if you can't get Advances at the cheaper tiers).
However, that would extend your journey time by quite a bit, especially on the way to Coventry, as the stopper is obviously slower than the express, and there's a wait of about 30 mins in the southbound direction for the express you'd have otherwise caught from Stockport (it's 'only' about 15 minutes in the northbound direction).