As you might expect, shopping carts come in a wide range of prices, from free to thousands of dollars. Does that mean that the more you spend, the better cart you get? Not necessarily.
There are several free shopping carts that are very powerful and full of features; for example, WooCommerce, a WordPress-based shopping cart, is free and offers the basic features you need to get started with ecommerce: unlimited products, connections to the PayPal Standard.
If you need more features than what is included with WooCommerce, you can buy plugins that extend WooCommerce. For example, you can buy payment gateway plugins for authorize.net, Paypal Pro, and many more. If your products have options that change the price of the product, you can buy WooCommerce Product Add-ons, and a plugin that makes is very easy to create product options for specific products or global options that you can apply to all products, to a product category
There are also several good shopping carts offered as a service. Shopping carts as a service means that shopping cart company hosts the shopping cart software on their server and you pay them a monthly feed to use it. Shopify is a shopping cart service.