Based on that average, we can easily determine how much more airtime you paid for by simply counting the number of individual calls you made during a given month. If you made 150 calls for example, that would use up, on average, an extra 75 minutes of airtime than if you'd been paying by the second.
However, just because you consumed an extra 75 minutes doesn't mean it will actually cost you anything. That depends upon whether you were going over your "bucket of minutes". Say for example your service came with 300 minutes, but you routinely used about 200 of those minutes. If that were the case, then those extra 75 minutes wouldn't have any impact on your bill at all. On the other hand, if you routinely used almost all of your minutes (or sometimes when slightly over), then virtually all of those 75 minutes are going to count against you. You'd either have to pay for them at the cost of overage, or you would need to move up to the next highest package available.
The only way to know for sure if charging by the minute is bad or neutral (because it certainly isn't good), you have to examine your bills and multiply the total number of calls by 30 seconds. Use that figure to see how much extra it would cost you, if anything.
However, being charged by the minute has a psychological impact that isn't obvious from simply working out whether it truly affects your bottom line. Knowing that you will be charged for a full extra minute if you go over a minute boundary makes you far more likely to become a clock-watcher during your calls (utilizing the minute minders that many phones provide). You then start to rush your calls when those beeps sound, and using the phone becomes a less enjoyable experience.