It seems whenever you click any button, including the girls' selection icons, the price of things in the shop is rerolled. Expensive upgrades can range between ~18-50ish gold and even low-cost ones are ~2-8, so this saves a *lot* of money in the long run. Helps a lot when trying to target-hunt that annoyingly elusive Rabbit Charm.
However, ever since buying proper Rerolls in the progression shop, i cannot actually leverage this... On top of the below-mentioned price increase each time the Shop button is selected, it looks like money is automatically spent to reroll the shoplist of whoever you last had selected upon even entering the shop. This ends up being such a vast resource drain that I've had trouble actually buying upgrades when i enter it with 40g only to have 31 of it burned away.
This is on 1.13, for version reference.