@model Decimal?
@{
    var postfix = "";
    if (ViewData.ContainsKey("postfix") && ViewData["postfix"] != null)
    {
        postfix = ViewData["postfix"].ToString();
    }
}
<div class="input-postfix" data-postfix="@postfix">
    <input asp-for="@Model" type="number" class="form-control" />
</div>