Rotating residential proxies are almost always billed per gigabyte, because the provider's own cost scales with traffic. Static ISP and datacenter proxies are usually billed per IP per month instead, often with unmetered bandwidth. Scraping APIs bill per request. Knowing which model you are on tells you what to optimise.
What counts toward the meter
Vendors differ. Request bytes, response bytes, headers, TLS handshakes and failed requests may all count, or only some. The vendor's documentation should say; if it does not, ask before you size an order, because on small pages the overhead can be a large share of the total.
Estimate your usage before you buy
Measure what one page costs you, then multiply:
curl -s -o /dev/null -w '%{size_download} body bytes, %{size_header} header bytes\n' https://example.com/
That is a plain HTTP fetch. A headless browser also loads images, fonts and scripts, which can multiply the weight of a page several times over. Blocking images and media in the browser is the cheapest optimisation on a per-GB plan.
On ProxyHive
Residential is billed per GB, from $5.50/GB for a single gigabyte, and the traffic never expires. ISP and datacenter IPs are billed per IP, and dedicated IPs have no bandwidth cap. The residential pricing explainer shows how per-GB rates move with volume and what else reaches the checkout total, and traffic expiry covers the other way a per-GB price can quietly rise.