Just because housing is cheap doesn't mean it's affordable — as a new study shows that nearly half of St. Louis renters struggle to afford their apartments.The study, conducted by housing search site Apartment List, found that across the U.S., the percentage of cost-burdened renters has dropped to its lowest point since 2008. (The site defines "cost-burdened" as those spending 30 percent or more of their monthly income on rent.)…