StoreHelper uses the old promotion system internally, and well, the old promotion system itself is marked as obsolete.

We would suggest to use the new promotion system, and you can use methods like this (from QuickSilver):

public IPriceValue GetDiscountPrice(CatalogKey catalogKey, MarketId marketId, Currency currency)
        {
            var market = _marketService.GetMarket(marketId);

            currency = currency != Currency.Empty ? currency : market.DefaultCurrency;

            var priceFilter = new PriceFilter
            {
                CustomerPricing = new[] { CustomerPricing.AllCustomers },
                Quantity = 1,
                ReturnCustomerPricing = true,
                Currencies = new[] { currency } 
            };
           
            var prices = _priceService.GetPrices(marketId, DateTime.Now, catalogKey, priceFilter)
                .OrderBy(x => x.UnitPrice.Amount)
                .ToList();

            foreach (var entry in GetEntries(prices))
            {
                var price = prices
                    .FirstOrDefault(x => x.CatalogKey.CatalogEntryCode.Equals(entry.Code) && x.UnitPrice.Currency.Equals(currency));
                if (price == null)
                {
                    continue;
                }

                var discountPrices = GetDiscountedPrices(entry.ContentLink, market, currency);
                if (!discountPrices.Any())
                {
                    return price;
                }

                return new PriceValue
                {
                    CatalogKey = price.CatalogKey,
                    CustomerPricing = CustomerPricing.AllCustomers,
                    MarketId = price.MarketId,
                    MinQuantity = 1,
                    UnitPrice = discountPrices.SelectMany(x => x.DiscountPrices).OrderBy(x => x.Price).First().Price,
                    ValidFrom = DateTime.UtcNow,
                    ValidUntil = null
                };
            }

            return null;
        }

        protected virtual IEnumerable<DiscountedEntry> GetDiscountedPrices(ContentReference contentLink, IMarket market, Currency currency)
        {
            return _promotionEngine.GetDiscountPrices(new[] { contentLink }, market, currency, _referenceConverter, _lineItemCalculator);
        }