Think about the volume you want to pack. Side bags + a top box will be about 80 - 90 liters of capacity. A top box + dry bag crossways on your rear seat will give you about 100 liters. A larger dry bag on your rear seat and on a rack will give you 100 - 120 liters.
Given the type of bike you have, how long you plan to keep it, the cost of side bags, racks, top boxes, drybags IMHO the best bang for the buck would be a large drybag crossways on the rear seat or an even larger drybag on the rear seat and a rack.
Given the type of bike you have, how long you plan to keep it, the cost of side bags, racks, top boxes, drybags IMHO the best bang for the buck would be a large drybag crossways on the rear seat or an even larger drybag on the rear seat and a rack.