Answer: 471
Step-by-step explanation:
Given : The population standard deviation is estimated to be $500.
i.e . 
If a 97 percent confidence interval is used and an interval of $100 is desired.
i.e. Margin of error = half of interval
i.e. E= 
Significance level : 1-0.97=0.03
Critical value for 97% confidence interval : 
Formula for sample size :

Hence, at-least 471 cardholders should be sampled.