Waterloo to Zone 6 on Oyster, off peak, is £4, so a round trip by train and 465 bus to Boxhill can be done for (£4+£1.50) x 2 = £11. The daily cap is £12.50.
You could, just about, do it for less than £9 by taking the Underground to Wimbledon (£2.80) and then two buses to Boxhill (£1.50 Hopper fare), although you would be cutting it fine to catch a second bus within the hour on the way back.