I used a combo of plain styrofoam and expanding foam (Great Stuff is fairly cheap). If slop some tile grout on the foam it greatly increased it's integrity and basically casts the item in a light concrete. Here's a link to a foam hide I made for my rat snake. The grout doesn't stick quite as well to the expanding foam but works really well on most other foams that don't have a gloss surface finish like expanding foam.
http://www.ssnakess.com/forums/enclo...n-working.html
Once the grout is in place (I used 3 layers total and allowed 24 hours dry time before each new coat) it's pretty impervious to the elements. I painted my deco with acrylic paint and then used grout sealer. If applying the sealer, allow that stuff to air out and degass for about a day. After that it's water proof, I make water bowls out of the grout and styro as well that have been in use for 3 months with no leaks.
You may not save a ton of money as the grout can be a bit expensive, but the sky's the limit on the design you can make. I can take pictures of the water bowls as well if you like. Best of luck.
Oh P.S. - You can make "vines" by taking some black silicone and coating a length of rope then roll or squeeze in some eco earth and let it dry.