Food is usually the biggest flexible cost on a long trip, and cooking is the lever that moves it. The saving is substantial and it is not automatic.
Staples are the trap in short stays
Cooking a first meal requires oil, salt, spices and other basics that cost far more than the meal itself and are left behind on departure.
Over a two-night stay that overhead can wipe out the saving completely, which is why self-catering pays on longer stops and rarely on brief ones.
Carrying a small kit of the items you use most, or choosing accommodation with a stocked shelf for guests, removes the problem where the trip involves frequent moves.
The saving concentrates on particular meals
Breakfast is the most expensive meal to buy relative to what it costs to make, and it is also the easiest to prepare with minimal equipment.
Lunch bought from a market and eaten outside costs little and requires no kitchen at all, which captures much of the benefit without any cooking.
Dinner is where eating out has the most cultural value, so many travellers cook the first two meals and eat the third out, which preserves the experience and most of the saving.
Shopping patterns differ from home
Without a car or a large fridge, shopping happens daily and in small quantities, which suits markets and small shops more than large supermarkets.
Buying for one or two days means less waste, and waste is what usually destroys the economics of self-catering while travelling.
Market timing matters too, since produce markets often close by early afternoon and prices frequently fall in the last hour of trading.
Kitchen quality varies far more than listings suggest
A kitchen may mean a full room with an oven or a single hotplate and a kettle, and listings use the same word for both.
Hostel kitchens are often busy at exactly the hours guests want them, which can turn a planned meal into a long wait for a hob.
Checking photographs rather than the amenity list, and reading recent reviews for mentions of the kitchen, is the reliable way to know what is actually there.
Cooking changes the trip as well as the budget
Shopping in local markets and cooking regional ingredients teaches things about a place that restaurant meals do not, since it involves the same choices residents make.
It also slows the day down, which some travellers value and others find an unwelcome use of limited time in a new city.
The honest assessment is whether an hour spent shopping and cooking is worth more than the money it saves, and that answer changes with trip length.