You will need to consult the catalogs. Most libraries will have a copy of Scott's Postage Stamp Catalog of the World and probably a specialized FDC cover catalog. These seldom take into account th cachet maker and variations.
First of all, you have to identify the specific stamp or card. That in itself can be a daunting task, but most of the catalogs are well illustrated with lots of information. Once you know what item you have, the catalog will provide a set of prices that can be used to estimate value.
You can go to eBay and see what prices they bring.
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
Scott Number US 1801You can consult a specialized US Stamp Catalog that lists First Day Covers in it. You can purchase the FDC for $2.25 from most dealers.
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.
They are roughly going to be worth $1 to $2 each. A few specific ones (high values) will be worth more. Your library will have a postage stamp catalog and there are online sites where pricing can be found.
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.