Depends how many coins you buy, but the best deal is buying 5,000 coins for 41$ (1 coin = 1 penny ingame).
Buying lapis is subject to digital sales tax depending on your location. For example in Louisiana, buying the 100$ lapis bundle (or 10k coins) costs something like 9$ sales tax.
So, if your location is taxed, it's something like 90$ for 18,000 lapis going through amazon, but you also get "coins back" with purchase, which is usually around 5% of the coins spent are put back into your amazon wallet (so final total is more like 85$ for 18k lapis).