A simple solution for a test of significance is a 1-df $\chi^2$ test of independence on a 2x2 table.
Row 1 is expensive products.
Row 2 is inexpensive products.
Column 1 is Group A.
Column 2 is Group B.
For directionality and magnitude, you can use a logistic regression predicting "expensive item" from group membership where the sample is all purchases. This would make it easy to get an odds ratio and confidence interval for the extent to which the buyer being a member of Group A increases/decreases the odds of the purchase being expensive.
(You can also generate the odds ratio directly from the 2x2 table, but the logistic regression has the advantage of a simple way to get confidence intervals.)