The Neewer 3-filter kit for the DJI Phantom 4 or Phantom 3 PRO or Advanced [incompatible with Phantom 3 Standard] goes for $12 in new condition, but if you are okay with used products, you can get it for $3.40 in “Used – Very Good” condition (cosmetic imperfections, original packaging) through Amazon Warehouse Deals.
Shhh! Don’t tell the Amazon bots. This is not an Add-ON item, so if you are a Prime member, you can get this with free 2-day shipping or select a slower shipping method for a “no-rush credit” (depending on whatever promotions the shopping cart may be running at the time of purchase).
If you are not a Prime member, you have the new-usual minimum of $25+ for free shipping.
This too was another find while checking the Warehouse Deals.