A 2.5 gallon or larger would suffice. Along with a heater, gravel, a cave, some live/silk plants (heavily planted) A thermometer... All this usually costs me about 50 bucks... or my pet stores just really over-price things. Lights aren't a must, by the way... I've found my bettas hate them. Bettas prefer to be in the dark because it de-stresses them. (But obviously give them daily access to light.) People have put lights over their bettas, though. That is just my opinion.
In the hobby for 9 years.