Now to the details:
Concrete is made of portland cement, aggregate, and admixtures. Portland Cement is cheap, and costs about $3.00 per 94 pound bag most anywhere. The aggregate is sand, and is also cheap.
Admixtures are chemicals and other things added to the concrete to give it various properties that you might desire. They include things like silica fume (for strength), superplasticizers (for strength and pourability), and accelerators (for strength and to help speed the hardening).
The admixtures that I use are superplasticizers, silica fume. Silica is cheap (about 20 to 50 cents per pound. It's used at 1/7 to 1/10 w/w of portland cement.) You can get by without the plasticizer, but the silica is readily available, and it's cheap. I'd reccomend using it.
I have tried making holds with only cement and varying ammounts of sand and water. I have not had much luck. The holds seem to break too easily. If you're not making incut or super positive holds, it will probably work fine. But don't expect to be able to do a dyno off a big positive hold made of just water, sand, and portland cement. It won't be strong enough.
This is not to say you can't make good holds with just sand, wanter, and portland cement. But to make positive holds strong enough, I really think you need at least the silica. It adds a lot of strength. The plasticizers make them stronger too, but the main advantage with them seems to be the convenience of pouring the holds.
Superplasticizers are polymers that reduce the ammount of water needed to mix the concrete, and thus makes it stronger.
Concrete is measured in pourability by something called a slump test. Concrete is packed into a cone about 18 inches high, and then the cone is removed. However far the concrete 'slumps' from the top of the cone is said to be the slump. No-slump concrete would be concrete that would move less than 1/2 of an inch when the cone is removed. This concrete would be good for making things like street curbs and bricks, but bad for us. We want to be able to pour the concrete into our molds.
There are 2 ways to make the concrete pourable, or increase the slump. One is to add water. If you add enough water, the concrete becomes more liquidy, and can be poured more easily. But, the more water you add, the weaker your concrete gets.
The other way to increase the concrete's slump is to add a superplasticizer. This allows you to make a concrete with a no or low slump, and when you add the superplasticizer, it make the concrete very liquidy (increases the slump) and pourable. The way this this works is that the plasticizer repells ionic charges in the concrete and thus makes is more 'slippery' (in a very non-scientific context.) Let's just say that if you've never seen it work, it's really cool.
Obtaining superplasticizers is another thing. They're not expensive, but good luck finding someone willing to sell less than 5 gallons to you. They come in 55 gallon drums and railroad tanker cars. I use about 5-6 ml per 5-6 medium sized handholds. Your best bet is to call around to different admixture and ready-mix places in your area and ask if you can have a pint. Bring your own container and maybe they'll be nice. If worse comes to worst, you can always buy a 5 gal drum for about $45 and sell off 4 gallons to your friends. You'll only need a little, and it has a shelf life of about 6 months according to the manufacturer. Good luck.