depends on a number of things i would say:
1. buy a set of the next highest multiplier to what you have
2. if you have a complete set of MEBC hats (the highest possible multiplier) then complete hat sets you have nearly finished
3. for completing the different rarity’s if you have none of each rarity i would go:
1st. common 21 hats= 420 pearls
2nd. uncommon 56 hats = 5600 pearls
3rd. Magical: 42 hats = 42,000 pearls
4th. rare: 140 hats = 70,000 pearls
5th. Legendary: 35 hats = 875,000 pearls
6th. Exotic: 126 hats = 1,260,000 pearls
in all you need 2,253,020 pearls
this order will change depending on how many hats you have and the sets you have finished
4. do you want to get the hats now or wait to see if you get them in a chest
5. if waiting to collect from a chest buy the exotic hats as these never (to my knowledge) appear in any chests.
6. buy by sets not by bird – so collect all 7 rather than collecting all reds for instance
7. how quickly you can collect pearls – for me 1000 pearls is collected quite quickly without spending money so anything up the the magical sets i could collect fairly quickly, both exotic and legendary take a long time to collect enough.
as you can see there is a lot to consider but i hope this helps you