I agree with stempy, if you're shooting for under 300gal. Then It'd probably be cheaper to buy.
Easiest way to look at it is; what would you charge per hour to build a tank for someone? Make an educated guess on how much time it would take, then double that (at least). Also, don't forget material cost and add at least an extra 25% for unforseen problems and possible forgotten needed items... after all that you will probably have a rough idea of the total cost of the build; notice I said rough idea. That way you can get a good idea on what you are willing to spend.
I'm a big fan of keeping an eye on Craigslist (daily). if you're a diy kind of person, then finding a good deal on a large glass or acrylic tank is only a matter of time. Building a stand, top, lights, or anything else not included in an online deal is fairly easy and not nearly as stressful or difficult as building the tank itself.
In the end, its up to you; thats just the way I look at it.