Auto discount prices based on quantities in Gravity Forms. Uses a percentage based approach.

Quantity Discounts

(function($){

	$.fn.quantityDiscounts = function(options) {
		
		// set up default options 
		var defaults = { 
			discountColor: '#FF0000',
			discounts: [
				{ limit: 174, discount: .75, text: "25% Discount Applied"},
				{ limit: 139, discount: .80, text: "20% Discount Applied"},
				{ limit: 99, discount: .85, text: "15% Discount Applied"},			
				{ limit: 79, discount: .90, text: "10% Discount Applied"},			
				{ limit: 59, discount: .95, text: "5% Discount Applied"}
			],
			test: "Test"
		}; 
		
		var options = $.extend({}, defaults, options); 
		
		return this.each(function(){
			var el = this;
			var item = this.id.replace('field','');
			var itemval;
			var discount = 1;
			var newprice;
			var curprice;
						
			$(el).on('keyup', '#ginput_quantity'+item, function(){
				
				// Set data element to keep original price in place
				if( $('#input'+item, el).data('original') === undefined ){
					$('#input'+item, el).data('original', cleanPrice($('#ginput_base_price'+item, el).val()));
				} 
				
				// More resets
				itemval = $(this).val();
				discount = 1;
				discountText = '';
				
				// Find discount and set text 
				$.each(options.discounts, function(){
					if( itemval > this.limit ){
						discount = this.discount;
						discountText = this.text;
						return false;
					}
				});
								
				// Discount display
				var discountDisplay = $('<span></span>').addClass('discountText').attr('id', 'discount'+item).css({'color': options.discountColor, 'padding-left': '10px'});
		
				curprice = $('#input'+item, el).data('original');
				newprice = discount * curprice;
				
				// Update fields with new price
				$('#ginput_base_price'+item, el).val(displayPrice(newprice));
				$('#input'+item, el).text(displayPrice(newprice));
				
				if( !$('#discount'+item, el).length ){
					$('#ginput_quantity'+item, el).after(discountDisplay);		
				}
				
				// Add discount notice
				$('#discount'+item, el).text(discountText);
			});

		});

		function cleanPrice(str){
			return str.replace("$", "");
		}
		
		function displayPrice(str){
			return '$'+parseFloat(str).toFixed(2);
		}

	};
})(jQuery);

Quantity Discounts with Amount

(function($){

	$.fn.quantityDiscounts = function(options) {
		// set up default options
		var defaults = {
			price_field: '#field_3_66',
			amounts: [
				{ limit: 100, amount: 65 },
				{ limit: 40, amount: 80 },
				{ limit: 20, amount: 90 },
				{ limit: 10, amount: 100 }
			]
		};

		var options = $.extend({}, defaults, options);

		return this.each(function(){
			var el = this;
			var item = this.id.replace('field','');
			var itemval;
			var amount = 1;
			var newprice;
			var curprice;

			$(el).on('keyup load', '#input'+item, function(){

				// Set data element to keep original price in place
				if( $('#input'+item, el).data('original') === undefined ){
					$('#input'+item, el).data('original', $(options.price_field+' input:checked').val());
				}

				// Get current value of checked field
				curprice = $(options.price_field+' input:checked').val();

				// Update pricing with pipe
				curprice = curprice.substring(0, curprice.lastIndexOf('|') + 1);

				// More resets
				itemval = $(this).val();
				amount = 1;

				// Find amount and set text
				$.each(options.amounts, function(){
					if( itemval >= parseInt(this.limit) ){
						amount = parseFloat(this.amount);
						return false;
					}
				});

				$(options.price_field+' input:checked').val(curprice+amount);

			});

		});

	};
})(jQuery);

this is exactly what I've been looking for. I tried putting it in my functions.php but it errors out. Is there another place that it should be placed? or should it be formatted differently?

It seems above code is a javascript jQuery.
you need to put it to a .js file.
but I think it can be hacked because it's a frontend code, what's your idea?