A First Day Cover (FDC) is an envelope cancelled on the first day of issue of a new postage stamp. Today they will actually say "First Day of Issue" in the cancellation. Originally they were just normal envelopes with the standard date bullseye type cancellation. Today elaborate cancellations and special envelopes are created to commemorate the events.
Scott's Specialized US Catalog lists prices for First Day Covers (FDCs) as well as plate blocks.
Yes
Yes
First Day Covers are worth more than ordinary covers. Stamps bought on the first day themselves, without cancellations, cannot be identified as being issued on the first day. Leave them on the cover if you have them!
N. C. Porter has written: 'Collecting modern first day covers of Great Britain 1971-81' -- subject(s): First-day covers (Philately), Catalogs
A lot of money.
First Day Covers without any address are preferred by many collectors. As long as the handwriting is relatively neat, how the address is affixed has little affect on value.
The price will depend on the edition of the stamps and the condition of the cover.
Cachet refers to a design printed on an envelope. It is often found on First Day Covers and Special Event covers. Before collectors got really interested in the postmarks, First Day Covers were identified simply by the cancellation. Early cachet covers are unusual and bring a premium. Later FDCs and events without cachet are going to be worth considerably less.
Not a great deal. The First Day Covers with the gold replicas are available for less than $4.
A return address is used in case the envelope is not delivered.
First Day Covers are valued at about a $1 each. You could probably sell it to a dealer for half that if it is in perfect condition.