Free proxies look tempting, but almost always it is a trap. Open lists live an hour or two: half the addresses are dead, the other half is slow. Useless for work, wasted time for money.
Worse: whoever controls a free proxy sees your traffic. Logins, passwords, card data — all passes through a foreign server. Trusting free proxies with data is a risk that does not pay off.
Free proxies for scraping are a separate pain: sites have long known public list addresses and ban them instantly. You get a captcha on the very first request.
If the budget is really small — better take a cheap paid pool with liveness checks than a free list. The price difference is pennies, the result difference is huge.