Colin Rosenstiel wrote

When I started working (part time) in London in 2001 an off-peak Day
Travelcard from Cambridge cost just 60p more (with Network Card
discount)
than a cheap day return to London Terminals. I guess the difference
without railcard discount was 90p.

That differential has risen steadily. In 2007 the undiscounted cheap
day
return and off-peak Day Travelcard fares are £18 and £22.50
respectively,
a difference now of £4.50, five times as much as 6 years ago. For
railcard users the difference now is £3.30, five and half times as
much
as 60p!

From January, FCC tell me, the CDR remains at £18 while the off-peak
Day
Travelcard increases to £24, widening the difference to £6, or £3.95
with
railcard discount. Meanwhile the Peak Travelcard goes up from £34.50
to
£36, the same increase in cash but a £1 less of a margin because the
SDR
goes up from £28 to £29.

Notice that also from Jan '08 NR Railcard (Disabled Persons, HM Forces,
Senior and Young Persons) discount will be available on Oyster capping
off-peak

Journeys in Peak Off-Peak Off-Peak (R)
Zones 1-2 £6.30 £4.80 £3.10

So, except for those with a Network Railcard or who want to use NR or
leave zones 1-2 while in London the TC add ons are not worth it and
Cambridge travelers should use Oyster PAYG.

I'm an Surrey SWT user and the CDR/ODTC difference has always been
(until the cash tube fare increase) more than two zone 1 cash tube
fares and thus could never have been as little as 90p in 2001. It seems
Cambridge started very cheap and ended up more than SWT.

The difference (from Walton-on-Thames) is currently £3.50 (after
11:30), £4.00 (after 09:30) and £5.70 (before 09:30).

--
Mike D