Ooh! I just love Harry Potter!
Designer wallpapers run anywhere from $60 -$250 or more per roll...here are some good sites for looking at ideas for wallpaper: http://decorationtrendy.com/harry-potter-bedroom-wallpaper-and-idea-decoration-design.html
http://www.designdazzle.com/2009/07/ideas-for-a-harry-potter-theme-room-4/
(I love the view of Hogwarts in this one, but you would have to click on the examples to see it) http://www.doorsonlineuk.co.uk/store/20-printed-bedroom-doors/654-printed-bedroom-door/
If you want your ceiling painted like the ceiling at Hogwarts, here some ideas: http://harrypotter.wikia.com/wiki/File:GH_Ceiling_POA.JPG
http://pkgsc399.files.wordpress.com/2011/01/harry-potter-enchanted-ceiling.jpg?w=460&h=282
The average cost to get your ceiling painted is 0.52 to 0.92 per square foot.
If you wanted the "floating candles" too (There is no Harry Potter room that may NOT have these!!) then check out this site: http://magnetoboldtoo.com/2012/11/11/harry-potter-party-hogwarts-enchanted-ceiling-and-floating-candles/
Apparently the candles are 2 bucks each, and she tells you how to make them...its so incredibly amazing and easy!
To buy the Hogwarts Wooden Steamer Trunk - Patronus Edition ($50), you can easily find it on Amazon. I think you really need this, cause this is a wonderful place to store the Harry Potter book series in!
Here are some sites to buy a Hogwarts rug ($26.95): http://www.carrollscove.com/harrypotter051.html
Dumbledore's Wand with Ollivanders Box is $35, and so is the Harry Potter Marauder's Map and a
Ron Weasley Lifesized Standup. The first two can be found on Amazon, but the third is found here: http://www.partysuppliesdelivered.com/ron-weasley-standup.html?utm_source=amazon.com&utm_medium=referral&utm_campaign=CSEs&gdftrk=gdfV21650_a_7c1379_a_7c4725_a_7cD461057#.UR_kyOhXuiY
You can get his broom for about $19, depending on which one you get. Tons can be found on Amazon.
Harry Potter Sign Metal Leaky Cauldron is about $21, and can be found here: http://www.fairyglen.com/60327.html#.UR_lQuhXuiY
The Witch parking Only sign can be found on the same site (its super cute, I personally think, but $39.99). It is 9 1/2 inches high, 16 inches across.
If you are a serious, serious, serious (I can not stress this enough) Harry Potter fan, then you must get the Triwizard Champion Wand set (17 inches high). You can find this on Amazon, but it is incredible to have..my friend has it, and it looks amazing!
If you want to fill up some room on your shelves, this lady tells you how to make potion bottles that are perfect for any Harry Potter fan...this varies in price, I suppose, but I'm sure this project won't cost much :): http://sewhooked.com/2009/02/23/harry-potter-party-revisit-1/
And last but not least, who can forget the famous Harry Potter Sorting Hat? You can find on Amazon for $20.47.
My, oh, my, I went all out, didnt I :) I'm not sure about what YOU want for your room, so I dont know the exact price...but I hope I gave you some sort of start :)
Have fun decorating! <3