wattage tells you the intensity of the light. A rule of thumb I was taught was 5 watts per gallon. BUT the rule doesn't always apply 100% of the time, just for easy to medium care plants.
I have a 2.5 gallon and it has 10 hrs of 60watt which is extremely intense lighting for something so small but I also fertilize daily and have weedy plants that grow super fast.
If you really want to save money, just buy a clip on lamp like this one
for $10 at Target, it work just as good, just either use a glass or mesh lid and pick a light bulb wattage of your choice.
If your starting out, maybe try getting some easy low light plants like cabomba and a 10w spiral bulb. I have cabomba, before I switched to the 60watts, it still grew nicely, now I am pruning every week and throwing pieces out because it grows too fast. lol