The title of this post might be a bit confusing but let's do the following thought experiment to compare prices for mobile Internet access with prices for circuit switched voice calling in terms of amount of data transferred:
A GSM voice call requires a constant data rate of about 25 kbit/s in both uplink and downlink direction (22.8 kbit/s with GSM channel coding to be exact), i.e. an aggregated 50 kbit/s. This includes overhead for coding and error protection in case you are wondering why I am not calculating with the nominal 12.2 kbit/s of the codec itself. So 50 kbit/s that is 3000 kbit/minute which is 375 kbytes per minute or 22.5 Mbyte per hour. The same amount of data goes over the air interface on the terminating side as well, so together that's 45 Mbyte per hour.
So how much are you charged for this amount of telephony "data"? Here things start to get difficult because prices vary per operator, country, contract, etc. So let's take a few examples. In Germany, prepaid calls are around 9 cents a minute. That is 5 euros 40 per hour, i.e. 5.40 euros for 45 MByte of data transferred. When staying in the same network, there are offers out there for 3 cents a minute, which would be 1.80 euros an hour. And if you take an on-net flatrate for 10 euros a month and then use it for 20 hours, that's 0.5 euros per hour, i.e. 1 euro for 90 Mbyte. To summarize the price for the 45 Mbyte of data transferred ranges from 0.5 euros to 5.40 euros.
Now let's look at the data side. A typical prepaid offer in Germany for 500 MB of data is 10 euros a month. In other words that's 50 MB for 1 euro and that is pretty much twice the amount as the cheapest voice example above. Let's go to the other end of the spectrum 20 GB for 90 euros a month. That is 222 MB per euro if used fully of course or 0.2 euros for the 45 MB. In other words, the cost for the data is about half of that of the cheapest voice call offer above in terms of amount of data transferred through the network.
So what's the point I am trying to make here? There is the perception in the industry that mobile data takes up much more network capacity than voice calls and brings in less money so voice calls must (still) be a much better source of income for network operators. But when looking at the calculations above that is not really the case.