There is an underlying fare filing for the main cabin and basic economy fares you are being quoted with each having a specific fare code. You can find the specific fare codes by clicking on the "Fare Rules" link when you quote a trip on delta.com. Airlines will make new fare filings from time to time, but they do not "dynamically" generate them on the fly. These are distributed to the various GDS systems and you can see a list of the fare filings that DL has on a given route with an ExpertFlyer subscription. In the case of JFK-LAX, the cheapest current fare filing for Basic Economy is VAVNA0BP. This is a roundtrip fare with a two-week advance purchase requirement for $236.60 (inclusive of all taxes). Basic Economy fares can be identified by the "B" in the second to last position in the fare code. The Main Cabin fare filing corresponding to this Basic Economy fare is VAUNA0MJ. Again a roundtrip fare with two-week advance. The price for this main cabin fare is $286.60 inclusive of taxes. Note the letter in the second to last position is an "M" which identifies it as a Main Cabin fare. The various restrictions on these fares can be found in the fare rules which can be brought up by clicking on the "Fare Rules" link on the fare quote. You can verify the rules and pricing with an ExpertFlyer subscription. Or, if you are cheap, you can bring up the fares for free on ITA Matrix and compare the pricing, fare codes, and fare rules. You can use the f bc=code advanced control on ITA Matrix to specify the particular booking class you are interesting in. (e.g., f bc=v would only show V fare class fares).