Billing Issue and False commitment from COX...

Hi All, I was having Internet promotional offer of $54.99 for 50MBPS internet package from Oct-2016-Oct-2017. On October 2017, I got a bill for internet of $59.99 (as my 12 months Promotion was over) for my Preferred 50MB internet speed . So I called Customer service to check this. That time she informed me that Promotional offer is applied to my account for next year as well and Hence she reduced the price back to $54.99 On November 2017, I received a Bill with Amount $79.99. When I checked with them, they told that offer is no longer available But I asked them to check call recordings. So they opened a ticket/Service Request KSR000006166258 to check and revert the billing issue. Initially they asked me to check after 2 Business Days (2 BD) but after 2 BD, they told me to check 5 BD. After 5th Business day, When I called them today, They transferred to some executive and She told me that Preferred 50MBPS Plan is discontinued, Hence she cant apply the offer.. This is ridiculous... She offered me 15MBPS plan for $48.99 or 100 MBPS for $69.99 and told me that above Ticket is resolved as my plan is not available henceforth. She forced me to changed the plan with Promotional offer. Now when I am checking promotional offer online, they are offering 15MBPS for $29.99 and 100MBPS for $49.99. So that means, COX guys are fooling their customer and charging more money for less service.. So is there any way, I can escalate as well as get good service???????