Depends on species.
I had a Tokay gecko, they are very large and nocturnal and make angry barks that sound like someone is squishing a duck.
Tokays are more opportunistic in their hunting than diurnal geckos, so they are more sedantary and don't require as much room.
10 gallon tank designed for reptiles, a drip watering system, daylamp and night lamp for basking,full spectrum flourescent light, very small under tank heater, orchid bark and live plants all ran me about $75.
Tank, heater, flourescent, and orchid bark were a 'tropical lizard complete combo setup' the basking lamps and plants were seperate purchases as was the gecko.
You will not need a night basking lamp unless you get a nocturnal species.
And if you get a desert species you will not need the bark or the plants.
As far as cost in the long run, that really cannot be calculated until you know the species, food requirements and frequency of feeding.
Overall though, unless a reptile is an herbivore, they are extremely cheap to feed.