The four common models
Most free-to-play economies use some mix of four things: time skips, premium currency, subscriptions and randomised rewards. Time skips convert money into waiting saved. Premium currency adds a conversion step that makes real cost harder to judge. Subscriptions bundle daily bonuses. Randomised rewards sell a chance rather than an item.
Time skips and subscriptions are generally the easiest to evaluate honestly, because you can see exactly what you receive. Randomised rewards are the hardest, and the ones we flag most often in reviews.
Working out real cost
Convert the premium currency back to euro before deciding. If a bundle costs 1,200 gems and gems come in packs of 500 for a set price, work out the per-gem rate rather than trusting the 'best value' label attached to the largest pack.
Then ask what the purchase buys in hours. Saving four hours of waiting is worth something; saving four hours in a game you will stop playing next week is not.
When spending genuinely changes the game
In competitive persistent worlds — King of Avalon and Rise of Kingdoms are clear examples — the top of the rankings is effectively reserved for people spending significant sums. That does not spoil the rest of the game, but it is worth knowing before you set your expectations.
In city builders and calm simulations, spending usually buys pace rather than power, which is a far more comfortable arrangement.