If you want to find a good determining factor for the cost of living and the general affordability of housing in San Marcos, CA, all you must look at is the percentage of monthly income that should be spent on housing expenses, otherwise known as the debt-to-income ratio (DTI). Generally speaking, no more than 28 percent of your total gross income should be spent on your housing expenses and not more than 36 percent of your gross income on total expenses, including debts. In San Marcos, CA, the median monthly income is $84,854 for those who own their own homes. Under the standard rule, that income level would allow for $1,980 to be budgeted for housing expenses each month, at the most. Renters, on the other hand, earn about $47,338, per the median income in the area. That would allow for renters to spend up to $1,104 on monthly rental expenses.