Paying more for an Ethernet cable can buy tougher construction, extra shielding, or a higher category rating, but it won't make the rest of the network faster. Devices negotiate a link based on the capabilities of both ends, so a cable rated above 1Gbps still tops out at 1Gbps when connected to two Gigabit Ethernet ports.
The actual speed is set by the slowest component in the chain—the internet plan, router port, network adapter, cable, or run length. Industry guidance from TIA shows Cat5e and Cat6 can handle 2.5Gbps and 5Gbps, and Cat6 can support 10Gbps over shorter distances. Cat8 is designed for data-center links over 30 meters, and Cat7 isn't part of TIA's North American standard, so a higher number isn't a reliable shopping strategy.
What's worth paying for is construction quality and verification. Copper-clad aluminum (CCA) has higher resistance than pure copper, which can cause heat and voltage issues in Power over Ethernet setups. Third-party verification, such as UL's performance verification mark, is a better indicator than brand. Legitimate cables can be cheap: Monoprice sells 10-foot Cat6 cables with pure copper and gold-plated contacts for under $5, and shielded Cat6A for under $10.