Answer
Females usually command a higher price because they live much longer than males in many tarantula species and a confirmed female removes the uncertainty of an unsexed spider.
Sex certainty has value
A seller needs reliable evidence to market an animal as female. Larger confirmed females may have years of growth, feeding and housing already invested in them.
Species and rarity still matter
A common female can cost less than a rare unsexed juvenile. Price reflects species availability, size, demand, growth rate and the confidence of the sexing result.
Ask how it was sexed
For a high-value female, ask whether sex was confirmed from a moult and keep any evidence or seller record supplied.
Do not assume from price
A high price alone proves nothing about sex. Buy against a clear scientific-name listing and an explicit sex status.