The one thing you left out is bolts and such hardware, I do not remember the exact number but on my build I spent several (i.e. more than 3) hundred dollars on grade 8 hardware (not to mention the ridiculous amount I spent on the raw steel.)
That is a very good point too. I bought my welder and the first thing I did was buy about $300 worth of steel for some projects. I used 2 of the 3 lengths of tube to build my sliders. That right there is $200 in steel plus the 1/2" grade 8 hardware which was probably in the neighborhood of $40.
Then you need to remember your consumables. An 11lbs spool of 0.035 Mig wire is 30 bucks. A refill on my 80CF argon tank is $60.
Then theres grinding wheels.
Flapwheels are $5-9 each depending on where you get them. Cutoff wheels are $2-3. Mind you those are the prices that most other people pay.
I found a website that is much much cheaper for grinding wheels and such.
www.bulletindustries.com
I ordered 3 7" flap wheels, 10 grinding wheels, 50 6" cutoff wheels $0.83 each, and 2 knotted wire brush cups. The total came to $72 shipped. They have flat $7 shipping too.