Allow an extra £100 quid
There are so many scams Ryanair pull, it's best to allow an extra £100 a passenger in case you get caught by them.
A bag can have 15kgs in it, and they *sell* baggage allowance, so if you paid for 3 bags, you think you have 45kgs but actually you paid for 3 bags totally 15kgs! If you had 45kgs of luggage and paid for 3 bags, your surcharge for excess baggage would be £450!
Then there's the non-transferable baggage allowance. You have 2 people and 2 bags, you have 30kgs allowance? No, you have 15+15. If you have 22+8, then the fee is £15/€15 per kilo, i.e. an extra £105. You have the same weight in luggage, and are travelling together, but there is still an insane fee.
There's the extra seat trick, pay for an extra seat and you get extra luggage allowance? No.
1a. You can check in a bag for a fee, but it will cost money
1b. You can travel with only hand luggage and it is free.
2a. You can check in at the counter and it will cost money
2b. You can check in on the web and it is free
Not mentioned: The combination 1b and 2b are mutually exclusive. If you have hold baggage you have to pay the extra checkin fee.
Really, I've been scammed by Ryanair myself, unless you're an expert, I'd allow £100 extra per person on whatever price they quote and just expect to be nickled and dimed out of extra money. Potentially a huge amount of extra money.