Most commercial 'cactus and succulent mix suck, but you could start with a commecial mix and add more drainage. The one I use contains 45% pumice, but I had to call and talk to the production manager to find that out. They claim it needs no amending, but for my climate and heavy winter rains, I need more drainage. Miracle grow C&S mix is really bad.

What you want is about 60-90% inorganic materials in the 1/8 to 3/8 inch size range. Gravel (yours is fine if it is not too big or too small), pumice, perlite, and turface are good ingredients. Sand is bad because it tends to compact and make you mix into cement. Peat is bad because it holds water far too long, and breaks down into smaller pieces that clog the pores of the mix.

Some folks use crushed redwood bark as the organic component. I use the 45% pumice commercial mix 1:1 with crushed pumice, ending up with about 73% pumice in my mix. There is some peat in there, but it has been diluted. This mix works very well for succulents like C. ovata, but I double the pumice (2 parts pumice + 1 part potting mix), kicking up the pumice concentration to 82% when potting cacti and other xeric plants. That is about as lean as I like to go, but some folks like it even leaner.

Check local stone/rock/landscaping suppliers. They will probably have a lean potting mix with little or no peat for far less cost than bagged commercial mixes. Also look for inexpensive gravel/red pumice/etc. while you are there. Usually these place are happy to sell small quantities of materials by the cubic foot (7.5 gallons, 100 lbs of gravel or 50 lbs potting mix) for about $5. Most commercial mixes will cost much more than that.

Hope that helps. Everyone has their own special mix, but the common theme is a high concentration of inorganic naterials in the 1/8 to 3/4 inch range. This ensures fast draining and low water retention.