There are many factors that determine value of a certificate. These include condition, age, historical significance, signatures, rarity, demand for the item, aesthetics, type of company, original face value, bankers associated with issuance, transfer stamps, cancellation markings, issued or unissued, printers, and type of engraving process.
Condition - The grading scale that could be used in stocks and bonds is shown below. Generally speaking, however, the grading is not used in the hobby as strictly as it is in coins and stamps. Most people acquire certificates for the artwork and history.
Age - Usually the older the more valuable, but not always.
Historical significance - What product did the company produce? Was it the first car, airplane, cotton gin, etc. Was the company successful? Was it a fraud? In what era (i.e. during a war, depression, revolution) was the item issued?
Signatures - Did anyone famous or infamous sign the certificate?
Cross Collecting Themes - Sports, finance, automotive, and railroad enthusiast interest.
Newsworthy - Some companies that are in the news (good or bad).
Certificate Owner's Name - Was the certificate issued to anyone famous or to a famous company?
Rarity - How many of the certificates were issued? How many survived over the years? Is the certificate a low number?
Demand for Item - How many people are trying to collect the same certificate?
Aesthetics - How does the certificate look? What is in the vignette? What color of ink was used? Does it have fancy borders or writing on it?
Type of company - What type of company was it issued for? Does the industry still exist? Has the industry changed a lot over the years?
Original Face Value - How much was the stock or bond issued for? Usually, the larger the original face value, the more collectible it is.
Bankers associated with Issuance - Who worked on the fund raising efforts? Was it someone famous or a famous bank? Is the bank still in existence?
Transfer Stamps - Does the certificate have tax stamps on it - imprinted or attached? Are the stamps valuable or unusual?
Cancellation Markings - Are the cancellation markings interesting to the item? Do they detract or add to its history and looks?
Issued or Unissued - Was the item issued or unissued? Was the certificate a printer's prototype usually stamped with the word "specimen"? Usually, issued certificates are more valuable and desirable.
Printers - Who printed the certificate? Was it a famous printer?
Type of Engraving Process - How was the certificate made? By hand? By wood engraving? Steel engraving? Lithograph? Preprinted form?
Paper - Was the paper use in the printing high quality or low quality? Has it held up over time? Does it have a watermark to prevent counterfeiting?