Hey Jason,
The cost of building a TD is going to vary depending on what materials you use, whether you come across them used or pay full retail, etc.
Here is a great thread from a member who built his entire camper for less than $300:
http://tnttt.com/viewtopic.php?t=30779
On the other hand, my camper will probably end up costing more than I thought it would initially, but I bought most of the materials full retail at Home Depot/Lowes and I had to buy some tools. Not counting the tools, it will probably end up costing me about $1500 before it is over, but I've stopped keeping track, actually.
Sometimes there is a dilemma, I could probably find a component less expensive if I waited and looked around for it, shopped the internet for the best deals, but if it is a Saturday and I'm building and I need it that day, then its Home Depot or Lowes. Some things you may want to splurge on, like a good vent fan or vintage style windows or hardware.
It is easier if you think of it as a 'payday' project, which means it costs a little bit every payday as you continue your build, a little here a little there, with a big chunk every now and again (for the trailer, or the 5x5 baltic birch, etc.). Usually, I don't have to spend a large amount all at once.
Of course, I'm just hammering nails (figuratively) there are some folks here who have built real works of art, and like anything else, one gets what one pays for. Some folks on this list have advocated well for spending the cost for quality.
Keep us posted as to your progress!
AL in Asheville
