without going into air purifiers, there are "cheap" ways to mask the smell:
1-hang sheets of fabric softener in and around your grow room. They don't give off too much off-gassing that will affect your plants and they absorb quite a bit of the smell. Combine this with a box or 2 of baking soda and you're pretty much good to go.
2-Create a mock exhast chamber for your grow room. Do this by having a light fan draw air from your grow room/closet and push that air into a show box filled with either fabric softener sheets or charcoal filters (aquarium filters). Have another fan draw the air out of the other end of the box. The air should come out smelling cleaner than laundry.
3- exhaust the air from your grow room to the attic or outside.
I, personally, play on the "cost-efficient" side and never spend more money than needed. Unless you're running a "commercial" grow op, there's no real need to spend a tonne of cash on equipment.