The cost of available plans vary by state, so I can't give you any specific information.
The basic Medicare is hospitalization insurance, almost like catastrophic insurance. It doesn't cover doctor visits and the care 99% of us use. You need a supplemental plan to cover non-hospital medical expenses unless you want to go out of pocket on everything. At age 65 that's probably not a good idea.
Generally Medicare allows insurance companies to sell 10 specific supplemental plans. F,G, and N are the best ones for most people, but you have to compare your needs with the plans shown here.
https://www.medicare.gov/supplement-other-insurance/compare-medigap/compare-medigap.html
Pay attention to what is and is not covered, Medicare has some bizarre rules about what they will cover. I'm fairly healthy, but I use a CPAP machine. I have a high deductible plan F that costs about $135/month. Medicare covers he CPAP machine and replacement parts, but doesn't cover other common things.
You might want to talk to
www.unitedmedicareadvisors.com. They seemed the least pushy about selling me something instead of educating me when I was learning about Medicare. I ended up talking to my company's health insurance agent, who helped me see the trees in the forest.
You'll also want Part D, which is drug insurance. I have Silverscript, which costs about $17 a month. The same pills that cost my wife $90/month cost me $3-$4.