Quick answer: None.
Cacti grow slowly, making them more expensive to cultivate. Optimal growing conditions for a cactus are temperatures of 80 degrees (F) and adequate (but not too much) water. One nursery I visited has a giant swamp cooler for it’s green houses to maintain the temperature at optimum growing through the summer.
Answer #2: Small ones.
If you buy small cacti, they are cheaper than large ones. Be smart–buy more small cacti and take good care of them and within a few years they will be as big as the larger ones for sale. I recently found some barrel cacti in four inch pots at a nursery for only $4.25! Anything under $10 is a good price.
Answer #3: Ones that propagate easily.
Prickly pear are cheaper than other cacti because it is easy to make new plants. Some agaves also make pups (baby plants) that grow out from the roots. Agaves that do this are cheaper than ones that don’t. You can probably get some pups from someone who has an agave in their yard, and voila! FREE landscaping!
I recommend getting getting lots of the free / cheap cacti and succulents to jump start your landscape, and then replacing them later on (or filling in some empty spots) with more expensive cacti.
Wrong answer: cacti from the desert.
It is illegal to move (steal) cacti from the desert. Since 1929 it has been illegal to damage native plants on, or remove native plants from, state land without a permit.
But I want some trees and ocotillos! Now what?
Some plants for your landscape don’t come in small sizes, but are foundational to your landscape design. Get the smallest healthy specimens you can find. Trees are worth spending a little more on because they take so long to get large, and are hard to replace.
Ocotillos are difficult. If you get the large, bare root specimens, you risk a low survival rate. I have bought 4 ocotillos so far and only 2 of them survived, even with immediate planting. Small ocotillos grow slowly, and over watering them changes their overall shape (more branching). So you can’t just water them extra to get them to grow faster.
Saguaros are another costly favorite. They are about $30 per foot. Specimens over 4-5 feet tall have a harder time rooting and acclimating to their new spot. I recommend getting a few 1-foot tall saguaros and watering them regularly (every 1-2 weeks). With this frequent water, they can grow 6-12 inches per year. If you are dying for bigger saguaros, buy them after you have put in the rest of your landscape. Spend your money on lots of plants first, then expensive plants last.