The US Postal Service website (www.usps.com) has an online postage calculator where you can type in the origin and destination zip codes to find out the exact cost.
It depends on a large number of factors, such as the weight, exact dimensions, and so forth. The safest thing would be to take it to the post office, where they'll weigh it and see what category it...
The cost would depend on the weight of the parcel. Big envelopes cost more than small envelopes of equal weight. You probably should go to your post office to mail this.