With Ryanair WILL it's best to assume that you are going to be ripped off as they will try to extract every possible bit of money from you that they can. They will "helpfully" add things to your booking that you don't need

The best place to start is Ryanair's own list of fees & charges

Let's start with "Frankfurt" to "Stockholm" (hint NEITHER airport is close to the city) on 21 Mar 2008 for someone who isn't savvy with Ryanair's thieving ways

Basic fare €9.99
Taxes, fees & charges €14.49
1 bag to check-in €9
Priority boarding €3
Insurance €12
Credit Card Fee €3

So your €10 flight now costs €52 and if you exceed the 15kg checked luggage allowance you get hammered for €8 per kg in excess - so someone travelling with 20kg pays another €40 on top of everything else

If booking multiple flights and/or multiple passengers then the CC fee is per passenger per flight - ie 2 people doing a return trip will pay €12 in CC fees not €3

You can avoid most of these fees however

Priority boarding and insurance are optional (though they may not seem to be) so that's €15 and you can use an Electron card to get free booking. If you have hand luggage only AND ARE a citizen of an EEA country then you can use online check-in so the cost goes down to €25