What is the cheapest Obamacare plan?
The age parameter is self-explanatory, but where you live and your salary may be less obvious. Health insurers calculate rates which are approved in each state. Even within one state, the same plan might have a different rate for people in one part of the state than another part. This is based upon how many healthcare providers are there, among other factors.
Last, your annual salary determines how much subsidy (government help) that you get to buy your health insurance plan. If you live in certain states and you earn less than 138% of Federal Poverty, then you would get Medicaid instead of a private health insurance plan. Medicaid does not charge any monthly premiums.
So, the cheapest plan is the one that works for your age, your state, and your financial situation. The person who will pay the least is young, lives in a densely populated state, and earns enough to qualify for subsidies but not too much to have to pay the monthly cost herself. (In some states, low-income people qualify for Medicaid which has no monthly cost. I suppose that is the cheapest, but I assumed you meant private health insurance rather than a public plan.)