Does it cost money to send a letter to England from America?

Yes his talent agency (that your letter goes to) is in the UK. This is the address:

Robert Pattinson Curtis Brown Group Ltd. Haymarket House 5th Floor 28-29 Haymarket London SW1Y 4SP UK

And as for requesting autographs, these are the guidelines:

If you want to request an autograph, follow the guidelines bellow. If you just want to mail a letter with the address above, and do not want anything back, then you can stop reading! If you live in UK send a properly stamped and self addressed envelope (minimum size 8.5” x 4”) with your request letter and a photo. You can include a piece of cardboard to keep the photo from bending in shipping and also add “Do Not Bend” on the envelopes. Send your letter and wait. On average, there is going to be a 3+ month wait for a response. If you do not live in UK, add several International Reply Coupons. The International Reply Coupons (IRC) will be used by the receiver to purchase english stamps. You can only get them at your post office. Place the IRCs into the envelope, not on it. For information on postage prices to receive a letter from UK, go to http://www.royalmail.com/

Source: http://www.fanmail.biz/33094.html

that depends on how you plan on getting his signature. if you try sending something with delivery confirmation to get a signature- that won’t work- they have assigned agents to handle that sort of thing.

now- if you are just sending a letter to him- of course you can mail letters overseas.

your local post office can help you with that - or usps.com.

good luck.