One more option for a budget buy this list under $50 is this one by Vinida.
Apart from this, the batteries are also easy to install and remove which makes it convenient for the users to replace their old batteries with new ones. It comes in a pack of two and is convenient to use. As only premium materials and advanced technology is used in making these batteries, it is reliable, charges quickly, and also has an extended durability. It is a Lithium-ion battery with a capacity of 3000mAh and a voltage of 18 V. If you are searching for an excellent Ryobi replacement battery under budget then you can opt for this option by Munikind.