Lured by visions of lower prices for natural gas, Becky Burger signed a contract with Spark Energy, an alternative retail natural gas supplier or “AGS.” But when the fixed-rate introductory period expired and the company switched her to a variable-rate plan that said her bills “may vary according to market conditions,” Burger allegedly wound up paying an average of 305% more than her local utility would have charged. In a class action complaint that contends the jacked-up prices weren’t based on market conditions — and …