To reduce the cost of a VPN, you should sign up for a long-term plan instead of a shorter plan. Usually, VPNs offer significant discounts for their annual plans compared to their monthly subscriptions — for example, ExpressVPN gives away 3 months for free on its longest plan.
Additionally, keep an eye out for promotional deals or coupon codes that can help you save money when signing up for a VPN service. The easiest way to find the best VPN deals is to bookmark and regularly check this page.