For teas from India, Sri Lanka, and Kenya, auction prices are often publicly available going back several years for various estates. If you add a fair mark-up to these to cover the cost of packaging and importing, you could probably come up with a baseline price for tea in various grades and forms from the various areas. If they are lower than the baseline for their group, they are inexpensive, if they are higher than the baseline they are expensive .
Tea from areas without an auction system in place are a bit trickier to gauge... in those cases, one has to go by personal assessment. What plucking standard was used for the tea? Increased plucking standards (2 leaves and a bud, 1 leaf and a bud, all buds, etc.) lower the yield which increases the price. Also, high plucking standards can't be mechanized very well, which also increases the price. Were the leaves hand-rolled or were they rolled by machine? Hand-rolling increases the price. Are the leaves whole, or are they in brokens, fannings, or dust? Whole leaf teas seem to be more expensive to produce, and more care needs to be taken in their handling. Are the teas organic? Organic cultivation tends to reduce yields, so the overall cost will increase. Are the teas Fair Trade? The fair trade premium is passed along to the consumer. Are the teas from an area with a higher cost of labor (Japan, Taiwan, etc.) or a lower cost (Kenya, Malawi, etc.)?
Are the teas fresh, or are they older teas that have been re-fired? Is there a high content of stalk, or is the tea in nice clean condition? If it is an aged compressed tea like Pu'er, has it been stored in high-quality conditions? Each year of quality storage increases the price for dark teas, but, most teas only lose freshness as they age, which should mean a decline in price.
How many middlemen has the tea passed through? Each middleman must make a profit, so the price increases with each level of "value addition".
Ultimately it comes down to making an educated guess, and many in the business world are reluctant to educate their consumers. As the old saying goes, "Never give a sucker an even break, or smarten up a chump."
Fortunately, through boards like this, through blogs, and through those who, while in the industry, are passionate enough about tea to see it as more than a means to an end, it is possible to become a more sophisticated purchaser.