Honest answer: it varies so much by country that any single figure would mislead you. The global pattern is consistent though. South and Southeast Asia, much of Africa, Latin America, and Eastern Europe sit at the cheap end, where generous tourist bundles cost about as much as a casual meal or two. Western Europe is mid-range, helped by competition and EU roaming. North America, Japan, and some island nations are the expensive end.
Structure-wise, most countries offer purpose-built tourist SIMs in 7, 15, and 30-day flavors bundling large data allowances with some local calls. These almost always beat standard pay-as-you-go rates and are the right default for short stays. For a 7-10 day trip, a 7-day pack plus one top-up, or a 15-day pack, usually covers it; for a month, take the 30-day bundle.
To get real numbers, check the prepaid or visitor pages of the destination's two or three largest carriers before you travel; they publish current pricing.