Are different booking classes on the same trip allowed?
 

I read the EduGuide and searched but am having difficulty finding a definite answer.

On multi-segment flights, does each segment have to have availability in the same booking class (fare code) to be bookable?

In case the above isn't clear, here is the situation:

Looking at the fare buckets on ExpertFlyer, there is a roundtrip flight with one connection each way. The first segment has availability in a $400 fare basis (and a $700 fare basis) and the second segment only has availability in a $700 fare basis.

Can I pay less than $700 for this trip because the first segment has available seats in a cheaper booking class?

To get the $400 price all segments need to be in the same booking class. If one segment is in the higher booking class the higher fare will apply. If its a round trip fare you can have one fare basis on the outbound and one fare basis on the return.

The only time you may have mixed in one direction is if you are stringing together 2 diferent fares. Say they are selling a T fare LAX-BOS and a V fare BOS-LHR you can combine them. In this case you will be charged whatever the base fare is for LAX-BOS + the base fare BOS-LHR + any taxes and fees

Do you average the two fares to get the net price?

If they are both round trip fare basis, its 50% of base fare in each direction + whatever the taxes and fees are.