I'm going to have to assume proper attire is a given, but no EDC. Hanks was on the island for around 4 years.

All prices found searching online or sourced locally, tax/shipping not included.

Tramontina Machete: $15
Stainless Steel Mora: $9
Sharpening Stone: $4

Bic Lighter x2: $2
Doan Firestarter: $7

Heavy Duty Ripstop Poly Tarp 10'x12': $12
Generic Hammock w/ Bug Net: $25
Wool Blanket: $20
Fleece Material, 2 yards $8
100ft Paracord or similar: $8

Canteen set (Canteen, Pouch, Cup): $7
Large Stainless Steel Pot w/ Lid: ~$5 at thrift shop
Copper Tubing, 5 feet: $6
Plastic Drop Cloth: $2
2 liter soda bottle with cheap swill in it (x2): $2
(Drink the cheap swill, use the bottles for storage. Might even find 3 liter bottles with no-name soda for this price at dollar store.)

Snorkel Set (Fins, Mask, Snorkel): $25
Gill Net: $7
.22 Rifle (Used) + 225 rnds ammo: $80 + $12

Multivitamin x500 (1 every 3 days): $19
1x Tube, Bacitracin Zinc Antibiotic Ointment: $2
1x Tube, Antifungal Cream: $3

$20 worth of rice and beans

Total: $300

Make a "pack" out of your wool blankets, tarp and cordage to hold your other items and meet the container requirement. If it gets chilly, use the wool blanket as an underquilt and cover up with the fleece, should cover reasonable south pacific temperature ranges.

The water setup above assumes there is a freshwater or at least brackish water source on the island. If there is not and you have to rely on rain and distillation of seawater, I would bring different stuff...

Pray you don't get sick... even if you get some fish antibiotics I doubt it will keep in the heat.