Not sure by "northern downtown" what you're actually referring to, but I use to park at the Calgary Place Apartments (900 block of 6th Ave SW) for $13 + GST for daily parking. It's ~$230 for unreserved monthly parking.
I also used to make use of
https://parkchamp.ca/; parking at Atrium I (corner of 5th Ave & 8th St SW) was $12/day with a "small car discount". Heated, underground, secure.
About the cheapest you'll find is along 9th Ave. The Continental Towers parkade (900 block) is $9/day. Obviously you're going to have a bit of a walk if you're working in 'northern downtown', but that's about as cheap as it gets.