Prices vary enormously by country, so I'd avoid quoting a universal number, but the general pattern holds almost everywhere: local prepaid data is far cheaper than roaming, and tourist packages bundle generous data with some calls for a fixed price over 7, 15, or 30 days.
In much of Asia, South America, and Eastern Europe, prepaid data is famously cheap and tourist SIMs with big or unlimited allowances cost roughly what one or two restaurant meals do. Western Europe, North America, Japan, and a few island nations sit at the pricier end. The best way to gauge value is to look up the official websites of the two or three biggest carriers in your destination and compare their prepaid or visitor plans before you land.
One consistent tip: airport SIM counters are convenient but usually charge a premium for the same product. If you can wait, official carrier stores in the city offer the identical tourist packs cheaper, and supermarkets or convenience stores often sell starter packs too.