An offpeak day return from East Croydon to Maidstone stations would be valid via Gravesend (where you could get off on your outward journey) or via Borough Green (where you could board on your return journey).
If you wanted to go via St Pancras and the highspeed line on the outward journey, the fare would be £26.10. Otherwise, the fare would be £23.20 (valid via London but not the highspeed line, so you could go Croydon-London Bridge-Lewisham-Gravesend on the outward journey, and return Borough Green-Victoria-Croydon). There is also a £17.10 "not via London" offpeak day return, but that's not going to cover what you need.
Do you have any railcards, or a London zones travelcard?