Without a doubt buy a bigger bottle if you are doing larger or more frequent water changes. If you are buying 50ml bottles for $4 you are wasting a lot of money, I'm seeing 2l online for $45.
I wouldn't go the cheap route with dechlorinator until you have a real good handle on what your water supplier does when & why. If you really want to be cheap about it, and aren't too worried about the ammonia created from the chloramine, pick up some sodium thiosulfate crystals. For somewhere around $10 you could dechlor anywhere from 30,000 to 150,000 gallons of water.