Well my opinions is that it depends on what exactly you want to keep. I recently added a 2.5 gallon tank, it has
live sand, live rock, a camel shrimp, two zebra dwarf hermit crabs and one domino damsel (temporary, will go into my damsel tank once he gets big enough).
As far as quipment, it has a heater and a flourencet lamp I bought at wallmart and a hang on the back filter.
Cost;
Tank =$5.00 (used)
Heater =$13.89+Tax
Light =$7.89+Tax
Filter =$17.99+Tax
Lid =$4.98+Tax (made my own)
Sand =$9.99+Tax (10 lb bag)
Rock =$1.89+Tax (per pound, was around 0.67 lb)
Salt Mix =$9.99+Tax (makes 50 gallons)
RO Water=$0.25+Tax (per gallon, 5 gallons was bought)
Ornament=$2.48+Tax (it was too cute to pass)
Shrimp =$5.49+Tax
Crabs =$0.99+Tax (each, 2 were bought)
Damsel =$2.99+Tax
Total of $92.62 with Tax