The last station in Zone 6 is Upper Warlingham, an Off Peak Day Return from there to Dormans is £4.05, however it is unclear from Stations Made Easy for [stn]UWL[/stn] whether or not there are Oyster readers that are accessible enough for you to get back on the same train (I suspect the photos were taken before Oyster PAYG was installed at this station). As there are exits both sides, it is theoretically possible, if you sit in the right part of the train, however I don't know this line very well so cannot advise on where to exit the train in order to get back on.
In the direction to London, if you failed to make it back on the same train, you could always walk to nearby Whyteleafe [[stn]WHY[/stn]] station (5 mins walk downhill).
Alternatively it'd be a lot less hassle to get a ticket from East Croydon (£4.90). Sorry this is too late for your journey.