I had my braces done privately and retainers were included in the cost, I didn't get charged any extra after treatment finished. I have fixed upper and lower retainers and other clear Essix retainers for wearing overnight. I presume if I broke or lost my retainers in the future I'd get charged for another set though.
If you don't want a clear Essix retainer maybe look into fixed retainers or a Hawley retainer which is more of a traditional looking retainer. Fixed retainers were a good option for me, you just have to make sure you clean them really thoroughly but they can be maintained by your dentist and are easily cemented back again if they do come loose plus you don't have to worry so much if you do forget to wear another retainer overnight occasionally. My Essix retainers don't look particularly long lasting, I'd give them maybe another year, I do clean them with retainer tablets but they still look a bit worn and I've had them since January or so. In terms of retainer costs I'm not sure, I'd check what was included in your orthodontic package before you panic! I imagine you're not looking at more than £150 though from what I've seen online.