Through return ticket = £19.70
Chelmsford to Stratford return = £13.10
Contactless or Oyster fare = £3.10 each way (unless you go via Camden Road/Camden Town, thus avoiding Zone 1, which costs £1.50 each way)
Cash fare = £5.90 each way (by either route)
So the cheapest way to do it is to travel via Stratford and Camden Road (and then taking the short walk to Camden Town) for a total of £16.10
Buying a ticket to Stratford and then travelling via Zone 1 comes to a toal of £19.30, which is just 40p cheaper than buying a through ticket. (That assumes that you use an Oyster card or a contactless debit card).
Sources:
http://www.nationalrail.co.uk/
and
https://tfl.gov.uk/fares-and-payments/fares/single-fare-finder