/// 
/// Given HTML overlay for an image in the store, render it.
/// [p:n] renders as price for item ID n
///  
/// Rendere         
         
You can only use the $1 notation if the replacement argument is a string, so you ended up passing $1 as a literal string to the int.Parse method.
Instead, use the (String, String, MatchEvaluator) overload with an anonymous method:
Regex.Replace(overlayHTML, pattern, 
match => FormatCurrency(GetItemPriceOnDate(DateTime.Now, currencyID, int.Parse(match.Groups[1].Value)))
)