I get most of my supplies- boxes, bags, delis, heat packs, cool packs, from Superior Shipping. If you are on their e-mail list, they have sales and free shipping frequently.
I get my Phase Change Material (PCM) packs from TCP Reliable. They are a brand called Cryopak. Donna Swisher is my contact. They have started offering a reptile shipping set, but I haven't checked into it yet. It's nice that they are responsive to our community, though.
I save up peanuts from items I order by mail.
I haven't added up how much a complete box with one or two PCMs, etc., adds up to. Hopefully less than $10!