Check out this warning from a typical spray can label: “Repeated or prolonged exposure to these chemicals without the use of a proper respirator can cause permanent brain damage, as well as damage to your lungs.“ But solvent- based spray paints aren't the only products that present a health hazard. You'll find similar warnings even on water-based spray paints and many insecticides.
The less expensive, N95 particulate filtering respirators (disposable paper masks with two straps) do a great job of filtering out particulate matter. But they don't adsorb toxic chemical vapors. To do that, you need activated carbon. Most $30 and up respirators have a P100 (100 percent efficiency and oil compatible) prefilter and an activated carbon filter for the chemical vapors. However, the $30 carbon filter respirators sold at home centers won't protect you against every single toxic chemical listed on every manufacturer's warning label. The bottom line is, any carbon filter respirator rated for solvents and paint is better than a disposable particulate respirator.
To save money over the long run, buy a respirator that accepts replaceable filter cartridges. But before you shop online, check out the availability of replacement filters at your local hardware or home center. You don't want to be searching every store in town for the right filter in the middle of a project.