How big the difference is really depends on what they cost you now.
I have 3 DC's - DS1 is 11, DD1 is 9 and DD2 is 2.
DD2 needed new shoes and boots last week (local shoe shop) - £108
DD1 needed party shoes before Christmas (John Lewis) - £18 
DS needed new Vans and boots - £89
So the youngest was the most expensive but I also choose to buy her those shoes instead of cheaper ones. That is rarely an option with DS as he is an adult size 7.
DS plays tennis and has stunt scooters/long boards - roughly £850 a year including kit and tournaments. Plus school trip at £375 and £60 spends = £1285.
DD1 goes to Brownies and swimming lessons - roughly £230 a year including kit. Plus 2 x brownie trips @ £75 each = £380
DD2 goes to playgroup, jiggy wrigglers and art club - roughly £400 a year plus book bug club (free) and soft play, day trips etc around £200 = £600
So yes DS is more but DD2 is still more expensive than DD1.
We find clothing balances out as all wear a mix from H&M, Next, Zara, M&S, John Lewis, Gap, Mayoral, Vans, Sarah Louise, Monsoon, Nike, North Face etc.
Plus DS's coat will be £140 but lasts him 2 years. The girls will be cheaper but they both have a play coat and a dressy coat each year.
I find the extra expenses are iPods, laptops, cinema trips with friends, pocket money, (soon to be phone!).
We also save for Uni each month.