I really don't mind ads popping up in my face in a game that I got for free. It also has to be done tastefully which implies triggering at a particular time or event. Usually I see one after I have completed the core game loop once assuming the time to complete a loop is reasonably long. I mean the game is free and if they can make some money off of ads via you as compensation for enjoying the game why not. It's hard for me to complain or argue about that.
Recently I was playing a free game which had ads and out of curiousity I went into the purchase screen to see how much it would cost to remove them. This is a game I really enjoy so the thought went through my mind. I was suprised with what I saw. It was $1.99 to remove the Ads as a separate item. I don't get any in game loot or anything, just the ads removed. I saw another option to pay $0.99 for some in game currency. I thought to myself why aren't ads removed if I purchase anything in their store. It's a win win scenario. I get some loot and no Ads which makes me really happy and they get some money.
Removing the ads also cost more than their cheapest item, is that because they know it's really annoying and most likely the option you will choose, so they should charge more? Obviously there can be a lot of arguement either way but I honestly feel that whenever I purchase anything from your store then I should automatically get the ads removed. It's a nice reward for your fans.