Cheapest and least hassle - I'm afraid it's a case of "pick one of the two", when it comes to Stansted!
You can sometimes get cheaper "Advance Open Return" fares directly through the Stansted Express website, but the price depends on how far in advance you're looking to travel.
Confusingly, unlike normal Advance fares, "Advance Open Return" fares are valid on any Stansted Express service within the ticket's date-wise validity.
The "Advance" part of their name only refers to the fact that these discounted fares are only available if you buy several days or weeks in advance - unlike normal Advance fares, which can often be available even just a few minutes before the train departs. No scope for confusion there!
Otherwise your next cheapest option is probably to get a (normal) Advance ticket to somewhere like Bishops Stortford and then to get a bus from there, or to get a coach from London Victoria or Stratford etc. Clearly that's much slower and less convenient, but if you're looking for the absolute cheapest option, that's probably it.