In order to estimate the value of the technology, you have to look at the target companies financials and the industry information. A licensee is only going to pay less than what they are making on the product. Its a guessing game at best. Look at the company's financial data from the various internet sources and try to ascertain the amount of money derived from that particular product. Look at the others in the field to determine their financial information.
Most licenses are short term because of the nature of technology.
Throw all the numbers together and try to find a number that's not too obscene. As a licensor, the licensing provides an opportunity to recoup the research and development costs involved in creating the technology. However, the cost to develop the technology is rarely a major component of the licensing value. As a licensee, you are seeking to license the technology if it provides a financial gain. If the price is too high, alternatives will be found.