Royalty free prices are determined by the size of the image used, where the images is used, i.e. an image used on product packaging will cost more than an image used one time, very small in a magazine.
The odds that you will have an image that is so unique that an image buyer would want to pay you thousands of dollars, or even hundreds of dollars for are very, very low.
As far as I know Alamy sets the prices for image usage, you don't set the pricing (The option to set the price if "you sell your copyright" to an image is available on some sites, but I don't think Alamy has that feature)
I believe that unless something is editorial (rights managed) or very unique and can be exclusive to only one image buyer (again rights managed) everything else should be set as royalty free. That's just my opinion.