too expensive

Wow, you can’t afford $5 per month? I’m guessing you’re not making any money from your site. You have the option to manually optimize each image individually, without the plugin. Yep, their descriptions says you’ll get 25mb for free. If you’re properly sizing your images, before using the plugin, then you should be able to get 25-50 images out of the free plan. Maybe more, depending on the size of the original. So, yes, the free plan will work for most. That is, if you know what you’re doing. Which is obvious you don’t.