Enlarge Image James Martin/CNET

Android is finally starting to bring in the big bucks for Google with its ad sales -- but still not as much as iOS does.

According to new indexes from Fiksu DSP, a mobile marketing company, ad costs on Android devices have more than doubled since 2015, steadily growing year over year. Marketers have become more willing to pay per user as Android's platform becomes more popular.

Android's growth is inching the mobile platform closer to iOS, but Apple is still the lead market for ad buys, as Google's operating system hasn't reached the same level of mass popularity as iOS has -- yet.

Apple did not immediately return requests for comment. Google declined to comment.

Despite the competition in pricing, a majority of mobile ads tend to run on both Android and iOS.

Google hopes that its latest Pixel and Pixel XL phones will help Android take on Apple, as the two companies battle for mobile supremacy.

"From last year to now, it's evident that both operating systems are experiencing changes that are largely a function of maturity," Tom Cummings, a vice president of new market development at Fiksu DSP said.

Updated at 9:40 a.m. PT: To reflect that a majority of mobile ads run on both platforms.