A straightforward return from Ricky to Sevenoaks with a Railcard is £15.
Ricky to Knockholt is £3.75 each way (£7.50 return), and a Knockholt to Sevenoaks Return is £3.30, a total of £10.80. With this option unless you are quick you'd be waiting for the train halfhour later at Knockholt.
Ricky to Orpington is the same price £3.75 each way (£7.50 return), Orpington to Sevenoaks Return is £3.90, a total of £11.40. With this option you could get the express train from Charing Cross to Hastings which arrives at xx20 and xx50 past most hours, touch out, and then wait 8 Min for the local train terminating at Sevenoaks.
Another option is a Z1-9 Day Travelcard + Boundary Zone 6 to Sevenoaks Return at £12.70.