Having used up about half of my starting 500 gems I tapped the little '+' icon next to the gem counter and got a swift lesson in F2P economics. Prices start at £2.99 for 500 gems and go up to - please, do brace yourself - £69.99 for 14,000 pretend jewels.
Seventy quid might sound like a lot - because it IS a lot - but, hey, at least that will let you dig out 56 entire squares on a map crammed full of the hardest variety of virtual rock before you have to start drumming your fingers and leaving the game running overnight while you charge your phone.
In what I can only assume was a sort of joke on the part of the developers, the top-whack option was labelled 'Best value'. It made me laugh, anyway. In a sense.
Now, I like playing games on my phone. I like Dungeons. I would probably enjoy Keeping them. There is however no scenario, no combination of events that doesn't involve my family being kidnapped in which I will spend seventy quid simply in order to play a 'free' game in non-geological time.
This kind of cynical, money-grubbing app should be easy to ignore. Both the App Store and Google play have many great games for which you can pay once and play forever. Titles like Minecraft or Grand Theft Auto: San Andreas that cost a bit to begin with but are then yours to play as often as you wish.
The trouble is, people do sometimes cough up for in-app purchases and if developers get the balance just right then the rewards can be immense. F2P games often have excellent presentation and usually offer at least a short burst of good gameplay in the beginning (oddly, Dungeon Keeper seems to have forgotten this part) ranking highly on download charts for this reason.
Gamers lose out, of course, with hobbled games offering thinner and thinner slices of entertainment in return for the time and money invested in them. With games like Dungeon Keeper, it is hard to see the sharp end of the F2P model as anything more than a scam at gamers' expense.
There is nothing intrinsically wrong with paying for extra content in a game but it should be just that - extras that give more enjoyment, not a temporary fix that makes the game behave like a game rather than a dismal cash funnel.