It looks like the built-in nigerian cultures have a straight english N
as their currency symbol. You can override that to use the Naira sign instead:
var formatter = new System.Globalization.CultureInfo("HA-LATN-NG");
formatter.NumberFormat.CurrencySymbol = "₦";
After this all formatting with formatter
will use the desired symbol, but be aware that the character also needs to be supported by the font the website renders in. Some fonts may not include the symbol or they might include a different symbol in its place (I have seen this on my local machine).