Invitations...help

It is not necessarily more expensive to have an inner envelope, just more old-fashioned. It also doesn't make the invitation more formal.

The more modern invitation manufacturers (and many of them are the very high end ones!) have stopped using inner envelopes because they aren't really necessary any more.

The purpose of the inner envelopes was originally to protect the invitation when they were hand delivered (think horse & carriage, etc.). The outside envelope had the name of the 'master of the house' and the address to deliver it to. It would get dirty in travel, and the butler would throw it out upon receiving it. The inner envelope kept all the components together, and the names of the invited guests were on that envelope.

With all the modern advances in todays world, the envelope doesn't get filthy in transport anymore, so only a single envelope is really necessary.

But some people like more old-fashioned, traditional feel of having the set of inner & outer envelopes, so that is why they still exist.