Where there is a reciprocal arrangement, they stay with you/you stay with them, then there is no need for money to change hands. However,I was brought up to always take a gift for the hostess. What, would vary, chocs, flowers, alcohol, items of luxury or unusual food.
At Christmas I would volunteer to provide a major food item; the turkey, gammon, cheese. wine. DS always turns up with a magnificent cheeseboard, much welcome with a family of cheeseoholics.
When our children were small we used to go and stay with friends intermittently and they with us and we would always give paper cash in an envelope. Back in the 1970s/80s the usual amount was £20.00 for the 4 of us.