When I listed this, I researched current listed Wal-Mart gift cards on Listia and this was the GIN I came up with. I don't know the answer to your comparison question.
At one point, the Marketplace Value for Ink Protocol (XNK) was 1 XNK per .012 USD so almost everyone that lists gift cards & whatever else based on dollar amount, started listing items as 1 XNK basically equals 1 penny. So 100 XNK would equal $1. Unfortunately, this conversion is constantly changing and now Listia's Conversion Rate is $0.018 USD per 1 XNK, so as of right now, 500 XNK would equal $9 based off Listia's conversion rate. Based off Marketplace Value 500 XNK would equal $7.88 at 0.0157 per 1 XNK. Can view the Marketplace Value Conversion at https://digitalcoinprice.com/converter/ink-protocol/usd/1