On Saturday, Healthcare.gov opened for enrollment in 2015 health insurance plans, and so far it’s proceeding without the horrific technical problems that greeted would-be enrollees last year. This year, as will be the case in future years, the enrollment window is just three months long. People can renew the coverage they had last year or choose a new plan. Those with incomes of between 100% and 400% of the federal poverty level (in 2014, $11,670 – $46,680 for a one-person household, $23,850 – $95,400 for a household of four) are eligible for subsidies to help them afford premiums.

The Obama administration has set a goal of having 9.1 million people sign up for coverage through the federal or state exchanges, or marketplaces (including people who are re-enrolling). That’s about two million above the current number of enrollees. They encourage current enrollees to shop again for coverage, rather than just automatically keeping the coverage they have.

As Wonkblog’s Jason Millman explains, it can be especially important for those who receive federal subsidies for premium costs to go through the shopping exercise again, because the subsidies are designed to make specific benchmark plans (the second-cheapest “silver” level plan in a location) affordable. Someone whose plan was the benchmark silver plan last year might find that there are now several cheaper plans available – so, that person will either need to switch to the new benchmark plan or pay a greater share of the premium.

People who were eligible for health insurance last year but didn’t buy it are getting an additional financial push to purchase it this time around. In 2015, people who don’t have health insurance and aren’t exempt from the Affordable Care Act’s individual mandate will have to pay a fee, which is the higher of two amounts: 2% of yearly household income, or $325 per person ($162.50 per child under 18, with a maximum of $975 per household). Next year, those figures will increase to 2.5% of household income and $695 per person; the fees will remain at that level but be adjusted for inflation in years after 2016.

Individuals are exempt from the mandate if the lowest-priced coverage available to them would cost more than 8% of household income; if they qualify for a hardship exemption (being homeless, being evicted, experiencing domestic violence, experiencing the death of a close family member, etc.); or if they fall into various other exemption categories, such as having income below the tax-filing limit or belonging to a federally recognized tribe.

The cost of plans – and, by extension, their affordability – varies by geography and by enrollees’ age and smoking status. The Kaiser Family Foundation’s Health Insurance Marketplace Calculator will let you try out individual scenarios to see 2015 premiums and subsidy amount (select “no” to question 4 on employer-sponsored insurance to see the most info). Vox’s Sarah Kliff summarizes findings from Avalere Health on 2015 premiums across the US:

A new analysis from health research firm Avalere Health showed that, on average, premiums for bronze plans (the skimpiest products) will go up by 4 percent next year. Silver plans (which offer middle-of-the-road coverage) will have a 3 percent premium increase.

But those averages mask huge variation. Average silver premiums are falling by 18 percent in New Hampshire — but increasing 28 percent in Alaska.  There are two states where average premiums are going up by more than 10 percent (Alaska and Florida) and two where they’re dropping by double digits (Mississippi and New Hampshire).

And even these state figures likely mask a lot of local variation, with premium changes likely varying from city to city.

In terms of how much Obamacare actually costs, that depends on where you live and how old you are. But as a benchmark, Avalere did calculate the average silver plan premiums for a 40-year-old man. In the states that use Healthcare.gov, they range from a high of $683 in Alaska to a low of $280 in Kansas.

Premiums are only part of the cost equation, though. Lower-cost plans come with higher cost-sharing. Someone who struggles to afford the premiums on a bronze or silver plan and develops a serious illness will likely find it even more difficult to cover the co-payments and co-insurance that accompany multiple doctor visits and expensive procedures.

Those who are worst off, though, are households with income below the federal poverty level in states that have not accepted the ACA’s Medicaid expansion. The ACA as written required all states to expand Medicaid to those with incomes of up to 133% of the FPL; because that provision covered the lowest-income families, the law made subsidies for exchange-purchased insurance available only to those with incomes at or above the FPL. The Supreme Court’s decision on the ACA made the Medicaid expansion optional. Currently, 27 states and the District of Columbia are implementing Medicaid expansions, with the federal government picking up 100% of the costs through 2016 and no less than 90% of costs for this population in all future years. In the remaining 23 states, a Kaiser Family Foundation finds that nearly four million uninsured adults are in this “coverage gap,” unable to get either Medicaid coverage or subsidies to purchase a marketplace plan.