General speaking, the value of a lighter can be identified with the price someone is ready to pay for it. This thesis would be correct if only the market would be perfect (perfect information and perfect competition). Of course it is not. That is why collectors search for a better way of valuating their trophies. The simplest way is to look for the most up-to-date reference sources, like ended auctions or on-line guides.
Estimating the value of vintage lighters is a very complex process. One need to take into account several factors. The collectors value depends on:
3. Brand, model, country, style of origin etc.
5. Quality and materials used for production
It is quite obvious that a lighter made of gold is more valuable than the one made of silver. The same counts to fine craftsmanship.
OK, are that all factors? Of course not, but these are the most important factors that must be considered in you estimation.
The nominal value of a lighter
PS. The Zippo Lady Bradford was manufactured between 1949-1951. In addition Zippo started in 1997 (production ceased in 2002) a reissue of the classic Lady Bradford calling it Lady Barbara. It is still available for approx. $100.
PS2. The yearly gold prices since 1793 can be found here.