The rates need to be recorded in the cottages table and then the correct rate should be copied from cottages to reservations for use in computing the cottage rental charge.
Cottages::__pkCottageID = Reservations::kf_cottageID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
Then Cottages::Rate should be defined as a number field with this auto-enter calculation:
Case ( Month ( Arrives ) < 5 ; Cottages::Rate1 ; Cottages::Rate2 )
I'm recommending using an auto-enter calculation so that if the rates change in the cottages records, existing reservations records for previous years will not change the rates charged. By only looking at the month, instead of the entire date, you will not have to update this calculation for next year's rentals.