Before convincing oneself that flight comparison sites dictate the budget, one must consider the threshold of relevance.
A cheap flight to Seychelles does not reduce the cost of a night's stay to €500 at the hotel, and that's often where the drawdown of the budget begins.
The fail-safe is to always evaluate the cost of living on site before validating the purchase mandate of the ticket.
Without this condition of comprehensive analysis, a "good deal" flight can turn into an overpriced trip, like buying a car at a good price and discovering that the insurance is exorbitant.