Depends on what part of Mt. Vernon and which garage/management company you’ll be working with. I currently pay $180/month (will increase to $195 in January 2023) with an SP+ property.

When I lived over there I paid 165 a month for parking in the bottom of my apartment building. In another apartment building in downtown, I paid 80 a month (A pre-covid special, that only went up to 135 or 145 I think? Cannot recall). Before that, I paid 204 a month at Saint Paul Plaza. I think the city posts the rates on their site. And the other private places tend to, too. Shop around, use Google maps to visualize the area, and you should find something.

I used to get Zipcars at 810 N Charles, for example: that has monthly parking iirc. It's a surface lot. I think the company is PMI, not sure.

Just moved here and went through the same parking ordeal. Symphony garage is $130 a month and probably the cheapest garage you’ll find, depending on where in Mt Vernon it may or may not be too far for your preference. If you keep your eyes peeled, rental property companies will post parking spots behind homes available for rent and they can be cheaper. I snagged one for $95 a month. It’s okay as a backup spot but in the week and a half I’ve moved here I’ve only used it twice and I’ve left the house nearly everyday. I say it’s only okay because it’s actually a tough parking spot to get into (very small lot with other cars that park like a-holes). Otherwise I’m able to find parking on the streets without too much trouble.