If I buy a cheap-day return from, say, Sawbridgeworth in Hertfordshire,
to London Liverpool St, it costs £10.50. However, the same station
will sell me a ticket with a one-day Travelcard included for £14.50.

It is a bit of an illusion, but also note our CDR takes you through zones
into Liverpool St, so if you were to compare fairly you should start by
stripping off the duplicated journey. So CDR to Enfield Lock and Z1-6. Or
CDR to Brimsdown and Z1-5 etc for each zone you go through. One of these
might work out cheaper still!

Obviously you have to weigh this up against the zones you'll need once you
get into London. I find a Z1-4 from my local station (Potters Bar) works
out the cheapest and most flexible, knocks about £2 off the price of an
off-peak Z1-6 travelcard. Shame the rail company don't issue it on one
ticket though, you end up with three bits of paper

Cheers,
Dave