Even nano tanks are quite expensive. I own a JBJ nano 24 and have spent several hundred dollars. If you're going to go with a biocube or a nano cube it has most of the stuff you need. In addition you will need live sand and live rock. Of course there are always extra costs like buying saltwater or mixing your own, fish food, tubing for water changes, acclimation buckets, fish nets, heater, thermometer....