Cheapest is a inline fan with carbon scrubber because it will last a lifetime other than sprays and gels.
I got my 6Inch inline W/ carbon scrubber for about $175? Most likely less, can't remember.
You only need a 4In Inline to do the job though, my tent only had 6in holes and didn't want to downsize.
If you can't spend at least the $125-150 on that, my next would say go with ona gel a nice tub of it for about $25-35.
You can just sit it in your grow room and it'll do the job, I live in a apartment as well the smell was bad at first used ona gel everywhere but inside the tent but when I did it worked wonders.
Also if you use a duct for exhaust in your grow room, I did some DIY and just took a cup of ona gel and put it in the ducting and swished the ducting around so the ona gel and oils coat the entire duct up, turn on exhaust and no more smell. (Best part is even when the ona gel is dried up in the duct, the smell still sticks to the sides and does wonders.