I'd like to go from London to Billericay on Saturday. Let's see if I understood the whole boundary fares thingy...
Liverpool Street to Billericay return costs £14.20. A Z1-6 travelcard costs £13.10 and a BZ6 to Billericay costs £7.20 so that doesn't make any sense if you're not travelling on the tube.
But if you're travelling within let's say Z1-4 the Oyster cap is £10.10. So it's £10,10 + £14.20 = £24.30 vs £13.10 + £7.20 = £20.30
Right?
Yes, this appears correct. In effect, what you are showing is that Liverpool Street to Billericay plus a Zones 1-4 Oyster cap is worse value than a Zones 1-6 Travelcard plus a Boundary Zone extension fare to Billericay
That's quite understandable in a way - as with the former option you are paying for the Liverpool Street to Boundary Zone 4 stretch twice (whereas with the latter option you don't double-pay for anything). The difference between the two options is approximately 'correct' seeing as a paper Liverpool Street to Boundary Zone 4 single costs £4.90 and the difference between the former and latter options is £4.00.
I hope you're aware there are bustitutions to Billericay this weekend though - you'll need to change at Shenfield for a bus. You could, if you wanted, use Oyster/contactless as far as Shenfield, as you would then be charged the cheapest available combination of zonal caps and extension fares to cover the travel you made. If you're an adult without any Railcards you're best off using contactless as the calculation is sometimes more favourable due to the way the system works.
As far as I can tell (please correct me if I'm mistaken
@MikeWh), the relevant extension fare to Shenfield, if the relevant zonal cap was Zones 1-4, would be £3.30 on the weekend. A paper ticket from Shenfield to Billericay would then be a further £3.90, so if my calculations are correct then this would cost you the same as the Travelcard+Boundary Zone fare option, but it gives you greater flexibility as you simply pay for what you actually end up using (as opposed to for what you
might use).