I agree with Rev.
I've been piddling and tinkering on things my whole life and love DIY in general. I own every issue of MAKE and spend my childhood in the family shop. There are a few tips that I've found to be true.
First of all, unless you truly enjoy the tinkering and figuring things out or you have a serious financial need to save money, don't bother. Its just not going to be worth it.
If you do enjoy it, then factoring in your time on the project is irrelevant. If you didn't spend all Saturday working on making some widget, Its rarely a real economic opportunity cost. You probably wouldn't have spend the time earning money or cleaning the house, you'd have probably spend it doing some other recreational activity that very likely would have cost you money.
So if the final results of your project is a cost of $100 plus 10 hours of labor, that's 10 hours you'd have probably spent losing money somewhere somehow anyway.
Always make sure you count the cost of all the little odds and ends. These things can be a killer on your budget and its all too easy to think of the project only in terms of the big pieces of material that go into it. I once built a soxhlet extractor from parts I found at Lowes and while the main device was mostly pvc with a heat lamp and a copper cold finger, all hooked up to a refrigerator compressor that was 'free' I spent almost an additional hundred on small copper and brass fittings along with some air hose fittings
If you have to order from several places to get the parts you need, consider shipping.
For instance I make my own hiking poles with $14 carbon tubes, some feet that cost a buck or two each, some foam grips that cost maybe $3 each and some paracord. They are a few ounces when done and much cheaper than anything at REI that's twice the weight but I end up spending more on shipping than on the actual parts.
Also, going in you have to think about the extras you know you're going to just HAVE to get while ordering that one part you need some somewhere. You start thinking that, "hey I'll probably get it sooner or later, I'll say one shipping if I just go ahead and pick it up with this order"
Eventually I want my own shop with a CNC router, and a RepRap machine among other toys and I could do a while lot with either of those. In general I don't consider having to purchase a tool as part of the project cost since it should be around a lot longer than the project takes. Some tools though are unlikely to ever be used again. If possible, borrow, rent or resell these if you want to save money. Another option is the disposable tool. I'm talking about something most likely from Harbor Freight. They do sell items that can last just as long as anything you'd find in Sears but in general the items you buy there will be starting to fall apart by the time you finish your project. Sometimes that's OK depending on how much you saved on the tool and how likely it is you'll need it ever again.
Be careful not to misjudge or you'll end up with a tool that works just fine but isn't quite what you need or should have purchased and you end up having to get an additional model.
A lot of the DIY appeal to me is that I can combine different ideas and create the ultimate super widget that all shall be envious of and stunned by my gadget making ability.
The reality it doesn't appear to have been done before there's probably a reason why. Don't get me wrong, this risk and experimenting is the fount of progress and development but there are a lot of failed prototypes along the way. If you're OK with that then you'll have a lot of fun even if it doesn't work the first 12 times, but if you want to make sure you DIY investment isn't going to be a failure, follow existing ideas and plans. In other words K.I.S.S.
In business resources are only valuable under certain circumstances. The most overlooked of those is that they have to be well organized. If you buy raw material, especially if we're talking about buying in bulk, you have to remember where you put the extra for the next project or you mine as well count ALL that material has being required for just the one project. If I print an extra form at work and don't need it, I trash it. I don't have a place for 'extra paperwork' or the room to make such a place and I'll never remember where I stash if it I just cram it somewhere but I know exactly where to find and print it again and the time I'll save doing so is worth more than the piece of paper I tossed out.
The following image pertains more to food but some of our consumables go bad as well. Glue goes hard after being opened. Tape breaks down on the role, some brake fluids start to go bad once opened, temperature and humidity can ruin many things before you get back to using it again.
All and all I've never regret anything I've made or tried to make even if most of the items don't work. That isn't always the point and at the least I learn a lot doing it. That is much harder to put a price on.