Sales Quotes - Unable to Update the Unit Price Field

Yeah, that is tricky one. 

Reason why your code doesn’t work for Sales quote is plenty of attributes, which applied to curyUnitPrice by Acumatica itself. Below goes one example:

        [PXDefault(TypeCode.Decimal, "0.0")]
        [PXDBCurrencyPriceCost(typeof(CROpportunityProducts.curyInfoID), typeof(CROpportunityProducts.unitPrice))]
        [PXUIField(DisplayName = "Unit Price", Visibility = PXUIVisibility.SelectorVisible)]
        public virtual Decimal? CuryUnitPrice { get; set; }

and that is far from the only one. I’ve tried initially to use CacheAttached, and remove section PXDBCurrencyPriceCost, but it didn’t work, some additional overridings may be needed to apply. So instead, I’d suggest to use one level higher, and go with RowUpdated:

    public class QuoteMaintExt : PXGraphExtension<QuoteMaint>
    {
        public static bool IsActive() => true;

        protected virtual void _(Events.RowUpdated<CROpportunityProducts> e)
        {
            var nR = e.Row;
            nR.CuryUnitPrice = 15;
        }

    }

on my machine worked perfectly fine: