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!