So the desired route is
St Pancras [STP] - Ashford [AFK] - Canterbury West [CBW]
Canterbury East [CBE] - Dover Priory [DVP]
Dover Priory [DVP] - Ashford [AFK] - St Pancras [STP]
I believe the Super Off-Peak Day Returns Plus High Speed (Restriction code F4, which has no weekend restrictions) from London Terminals to a variety of destinations including CBE/CBW, DVP, Ramsgate [RAM] and Deal [DEA] all have the same price, £41.20. Fast trains travel both STP-AFK-CBW-RAM and STP-AFK-DVP-DEA-RAM.
It seems the £41.20 ticket to DEA (or any other station between DVP and RAM) is valid both via DVP and via CBW-RAM (due to DVP and RAM both being valid routeing points). Hence, a ticket for London Terminals to DEA can be used for STP-AFK-CBW outward, and DVP-AFK-STP return. Here's a Trainsplit search for next Saturday, forcing changes at CBW out and DVP return:
View attachment 161747
You then just need a separate £10.50 Anytime Day Single for CBE-DVP (unless you spend an extra 40ish minutes going via RAM for free). That makes a total of £51.70.
(I initially thought a ticket to RAM would also work since journey planners will validate STP-RAM via DVP, but then I realised they only do so if you don't change trains at DVP, so that's just the direct train rule obscuring the lack of a mapped route from London Group to Ramsgate Group via DVP.)
P Binnersley's suggestion of a Canterbury return together with a Canterbury to Folkestone Central [FKC] Off-Peak Day Return (Route Any Permitted) would be £41.20 + £16.60 = £57.80.