How to value your side business

Hello Everyone,

I am a software engineer(backend mainly)with 5 YOE working with Python, working at a US based IT company. For most of my career i have worked remotely (just one month of going to office)

During COVID, I started working with a client as a freelancer where developed a tool (home page picture attached) for him to do Cryptography trading at websites like Paxful and Noones. Initially we faced a lot of challenges with banks, but now we have steady the ship and doing on average $258,000 monthly since Apr 2023. I am getting a royalty of 0.5% on total revenue done montly for setting up the business and for maintenance. Any other work I do with him i charge $25 per hour.

Now, my main question is how do I value this business??