Calculating business logic

There's two things that I'm seeing right off the bat.

1. Don't use "magic integers" where you can be using Enums. GetDiscounts should probably receive an enum, not an int:

   var discVal = GetDiscounts(items, RecordEnums.ProductClassification.Service);

2. You don't need to Select then Max. Just using Max will give you the desired outcome.

   decimal highestContractTerm = items.Where(x => x.ProductTypeId == (int)RecordEnums.ProductClassification.Service).Max(y => y.RecurringTerm ?? 0;

It is not bad. Here is my shot at it:

private static decimal GetRecurringCostEquiv(List<QuotationItemViewModel> items)
{
    var discVal = GetDiscounts(items, (int)RecordEnums.ProductClassification.Service);

    // This can be calculated using its own small function
    // Consider using SQL-like LINQ syntax and split it into multiple lines.        
    decimal highestContractTerm = items.Where(x => x.ProductTypeId == (int)RecordEnums.ProductClassification.Service).Select(y => y.RecurringTerm).Max() ?? 0;

    if (discVal <= 0 || highestContractTerm <= 0)
    {
        return GetCurrentRecurringCosts(items);
    }

    // The following 2 lines could be its own function.
    // Again, consider splitting LINQ into multiple lines.
    var calc = items.Where(x => x.ProductTypeId == (int)RecordEnums.ProductClassification.Service).Select(y => y.RecurringCostCore * y.Quantity);
    decimal total = calc.Sum() ?? 0;

    // Get rid of the totalOverTerm variable and bring its value into the second line
    decimal totalOverTerm = total * highestContractTerm;
    decimal totalOverTermMinusDiscounts = totalOverTerm - discVal;

    // Do this instead:
    // return Coalesce(totalOverTermMinusDiscounts / highestContractTerm);

    decimal? result = totalOverTermMinusDiscounts / highestContractTerm;
    return result ?? 0.00M;
}

// Consider using this function
public static decimal Coalesce(decimal? nullableValue, decimal valueIfNull = Decimal.Zero)
{
    if (nullableValue.HasValue)
    {
        return nullableValue.Value;
    }

    return valueIfNull;
}

Try this. There are several areas that are hard to comment on due to the null-based nature of several of the variables you are utilizing. Also the static helper methods on the bottom need to be refactored/replaced to conform with the methods you already have.

    private static decimal GetRecurringCostEquiv(this List<QuotationItemViewModel> items)
    {

        #region Variable Acquisition

        var products = items.Where(q=> q.ProductTypeId == (int)RecordEnums.ProductClassification.Service);
        var highestContractTerm = products.Max(q => q.RecurringTerm).Normalize();
        var discounts = items.GetDiscounts();

        var hasContractTerm = highestContractTerm > 0;
        var hasDiscounts = discounts > 0;

        #endregion

        if (hasDiscounts && hasContractTerm)
        {
            decimal cost = products.Sum(q => q.RecurringCostCore * q.Quantity).Normalize();

            return ((cost * highestContractTerm) - discounts) / highestContractTerm;
        }
        else 
        {
            return items.GetRecurringCost().Normalize();
        }
    }


    private static decimal? GetRecurringCost(this List<QuotationItemViewModel> items)
    {//TODO: should be reviewed/refactored according to the base function
        return GetRecurringCost(items).Value;
    }

    private static int GetDiscounts(this List<QuotationItemViewModel> items)
    {//TODO: should be reviewed/refactored according to the base function
        var discounts = 0; //Do Stuff

        return discounts > 0 ? discounts : 0;//ensure discount value compliance
    }


    private static decimal Normalize(this decimal? value) { return (decimal)value; }

    private static decimal Normalize(this int? value) { return (int)value; }