Yes, this hobby does cost some money...

Without the correct amount of postage on your envelope, your letter may be returned or the celebrity will have to pay the excess amount to get the letter; it is difficult enough to write to a celebrity and get a positive response without them paying the postage.  Along with that, postage on the returning envelope (the SASE) is important in that if the celebrity does send you back something, I'm sure you would want to make sure it gets delivered.

Depending upon your post office, they may deliver the returning envelope with insufficient postage but, they will leave a bill for the remaining amount to be paid by you or you may just get a note stating that they are holding a letter for you until you pay the remaining amount.

The first chart below will show the approximate weight of items that you are sending.  Use this chart to calculate how much the total weight will be and use the second chart to figure out how much postage is needed to mail it.  I have compiled this list from using the rate calculator at the United States Postal Service web site which calculates both domestic and international postage amounts.  Although I have weighed the items myself, you should use these weights as approximate.

Domestic postage (from one US address to another)
The third chart shows the amount that you should use on the SASE.   For a standard size envelope, the first ounce will cost $0.34 and each additional ounce is $0.21 cents.  For any envelopes larger than a standard size (and over an ounce in weight), the initial postage is $0.55 with each additional ounce costing $0.21.

International postage (from a US address to an address outside of the US and it's )
US postage will work on letters mailed to the address outside of the US but, US postage cannot be used on the SASE.  There are three ways to pay for postage on the SASE:

1. Apply stamps that are from the country that you are mailing to.  You can write to the main post office of the country and purchase stamps from them.  I haven't tried getting stamps this way so you are on your own on this.  Here are a couple of addresses to get more info about purchasing foreign stamps:
   
   UK postage:
   Royal Mail Direct
   20 Brandon Street
   Edinburgh EH3 0SP
   SCOTLAND
   
   Canadian postage:
   National Philatelic Centre
   75 St. Ninian St.
   Antigonish NS B2G 2R8
   CANADA
   Phone: 1-800-565-4362 (you can purchase stamps with a credit card)

2. Use IRCs (International Response Coupons).  These are small coupons (see picture) that can be purchased at most US post offices.   They cost $1.05 each and are redeemed at foreign post offices for minimum postage.  Each IRC will cover about 15 grams (a little over 1/2 ounce) of airmail postage. This is probably the easiest and most common method to use if your post office carries them.   If they do not, ask them to order them.  As a rule, it is always good to send at least 2 IRCs for a standard sized SASE and 3 IRCs for a 9x12 SASE to ensure that there is enough for return postage.

3. Send cash.  This is more risky because you may never see your money again.  IRCs may not even be redeemable in some countries (I believe the German Postfache  - post office - doesn't) and if you can't get foreign stamps, this may be the only way.