Piggy Bank

The Piggy Bank is a mechanism in Hero Wars (browser confirmed) that incentivizes "reactivating" accounts (a login + server combination) that have made purchases in the past, but have not done so recently, and being most likely based on the VIP level that it appears the deal is "often" good. NOTE: conversions, maximums, and prices may vary by VIP level and can be "adjusted" by the developers without notice :( so prices, conversions, and maximums are not gauranteed, and are likely to be different when/if you see this on your account.

Appearance
the piggy bank appearing is linked to the account having VIP greater then 0, and that the account has not made a purchase in at least 30 days (this is probably to prevent getting the piggy bank on any account that makes purchases during 4x sale). Other triggers/conditions/thresholds may also exist, but testing is not ideal.

once completely filled the Piggy Bank will only have a "short time" to be used (24 hours) before it goes away until it is triggered again, but it is unclear is the triggers/conditions/thresholds are different or not for it to reappear.

Filling the Piggy Bank
when the piggy bank is presented to the player it will have a conversion from energy into "saved" emeralds

For example: At VIP-2 the conversion rate is 1 energy to 4 Emeralds with a max Emeralds of 5,000

At VIP-10+ the conversion rate is 1 energy to 25 Emeralds with a max Emeralds of 15,000 (this conversion and maximum have been noted by VIP10, VIP13, and VIP15)

Once completely filled the player will be given a 24 hour timer to "open" the Piggy Bank for Real Money

Opening the Piggy Bank
once the Piggy Bank has been filled to a minimum threshold it can be opened for an amount of Real Money rewarding the emeralds accumulated and also giving some VIP-points (relative to the amount of money spent as normal) it was previously reported that at VIP10+ had a piggy bank offer for 1 energy to 25 emeralds, max 12,000 costing 9.39 (this is shown as an example of the potential for these things to change)

Pricing and comparisons to 4x
most recent information (Nov 3, 2021) on Pricing for these 2 examples shows

VIP2 costing 7.49 USD for 5,000 emeralds (667.6 emeralds/USD); a VIP2 account during 4x salle at 250 USD for 155,150 emeralds (620.6 emeralds/USD)

VIP10+ costing 11.29 USD for 15,000 emeralds (1328.6 emeralds/USD) a VIP15 account during 4x sale at 250 USD for 319,000 (1276 emeralds/USD)