Depends on your needs. Ipv6 proxies are extremely cheap, but they will work with 6-7 big websites in general - Facebook, Google, YouTube, Instagram, Telegram etc. So if you need proxies for another goal, then you can purchase Ipv4 datacenter ones if you are looking for cheap proxies. They are not as safe as residential or mobile agents, but if you need a cheap option then IPv4 proxies will suit your needs. If you need proxies for FB for example, you can even try IPv6 ones, they are the cheapest. You can check proxy-sale seller for example, you can find both types there