First let me say that I never had any intentions of building thrifty. I'm not one to scrimp and hunt for bargains or "found opportunities". I want to design and build with purpose, so buying or custom building things to fit my vision comes before waiting for opportunity to knock or patching odd things together.
There seems to be a consensus that most people run over budget by at least 20/ct. This seems to hold true whether you plan for a $2500 build or a $7500 build. At the moment I'm more like 30/ct over my high end estimate, but (hopefully) all of the big ticket items have already been purchased.
That said, the off road style suspension, wheels/tires, and axle with brakes really added up on mine. The four 2x10 x 10ft "select" (clear straight grain) cedar boards that became my internal framing were pretty dear at $600, but I have no regrets; not a single knot worth mentioning, light weight and rot resistant. Something like 13 or 16 sheets of 5mm marine spec Okoume plywood, with shipping; no regrets, except I would build 60 inches wide instead of 64, and might even buy more sheets to have less seams if I were to do it again. The foam was a good chunk. I bought canvas and extra glue, then changed my mind and decided to go with epoxy and fiberglass for my outer skin instead, north of $500 for composite materials (I'm thinking this is about the same as what most spend on aluminum and trim).
$200 for windows, at least a few hundo for hardware, and on and on.
Extra lumber for templates, jigs and fixtures; you can't always count on having the right piece of scrap on hand, so sometimes it takes an extra trip to the lumberyard.
The Maxxair Deluxe roof fan was a big dollar item, too. All of the wiring, even for a simple electrical system can add up, especially when you start buying multiple rolls of different colored wire; and while the modern LED lights are amazingly affordable, they start adding up when you use a bunch of them.
Everything else can nickel and dime you, too. $20 worth of chip brushes, $60 in sandpaper, $100 for three gallons of wood glue, then add stains, polyurethanes, paint, masking tape, etc. These things all keep the funds trickling out. If budget is tight, it is better to sharpen your pencil and look at everything as closely as you can... then triple it.
Tools. IMO, to build a quality TD you must have a good solid table saw with out feed table/workbench, and a power miter saw doesn't hurt either. You can get by w/o these, but you will have to work much harder to get the same results. Assuming you have these in place, or plan to work around not having them, there will be at least 2 or 3 router bits that you are likely to want to add to your collection before your build is done. Those flush cutting bits, one with a top bearing and one with a bottom bearing, will really come in handy. I also added a rabbeting bit with a separate bearing set, a guide collar setup, several different size round over bearing bits, and a trim router to my router collection.
People will poo-poo me for counting up all of the consumable supplies and "extras", but when someone comes on here and expresses concerns about affordability and lack of funds, it is not fair to omit these "hidden" costs. Sure, there have been some truly budget builds, and if you keep things simple you can have an affordable camper, too, but then again it seems that most people don't keep accurate accounts, so they forget to include all of the smaller stuff when they add their build costs up in their heads. "The best way to build an affordable camper is to not keep track."
Here's one last thought: I build at a friends shop, about 3-1/2 hrs extra driving time per week. It wouldn't surprise me if I have spent more money for gas commuting to the build over the last 3 years than many people spend on their whole project.

Consider all of this before you build, but if you are looking for a fun creative project that you can build with pride, don't let budget stop you. Build what you can with what you have, just plan it well and stick with it. You can make it happen if you work for it!
