Always buy the cheapest tickets exclusively on the airline's website. Domestic flights are very cheap, for example one-way tickets from Delhi to Mumbai rarely go over 37 eur.
IndiGo fly all over India, with every major city in their network. In addition, connections are often very frequent (usually 3-5 times a day). The best time to buy a ticket is any time up to 3 weeks before departure, then the cheapest seats can sell out quickly.