How much $$$ to send envelopes?

i need to send 6 10"x13" paper envelopes to california (i'm in indiana). Inside each envelop is a 9"x12" paper envelope that contains a piece of light cardboard, a note, and a photo (this is the same for all the 6 envelopes. i need to put stamps on the 9"x12" (so my reciver can send it back to me without having to buy postage) and i need to stamp the 10"x13" envelopes to get them to california. i need to know how much this will cost, i'm stamping 12 envelopes in all. i know its not possible to give an exact amount because it is based on weight, but i'd like to know if this will be closer to $10 or more like $30. anybody who has experience sending large envelopes would really be helpful. i have never done it before. thanks!