How much does it cost to purchase a barcode?

It depends on which kind: barcodes for internal use cost nothing — you can generate Code 128 or QR codes for your own SKUs free, in unlimited quantities, no license required. The barcode format itself isn't owned by anyone.

What costs money is a retail UPC/EAN, because those must be globally unique and are issued by GS1: a single GS1 US GTIN costs around $30 as a one-time fee, while a company prefix (for many products) starts at a few hundred dollars initially plus an annual renewal. You need GS1 codes if your products sell through retailers or marketplaces that require registered UPCs (Amazon, most grocery).

Avoid third-party 'cheap UPC' resellers — marketplaces increasingly verify ownership against the GS1 database, and codes bought from prefix resellers can fail that check.