Ask HN: A company wants to buy me a perpetual license, how can I value it?

I'm a solo business with a SaaS (It is a software to process data, I prefer not to reveal the name) and one of my clients, the bigger one pay me around $1800 per month for the service, my client wants a perpetual license (they are a large company and need to keep control over it) how can I set a value for it? My net profit for this client is only 15% (I pay AWS costs to keep the service running), because a bad negotiation with a 3 years contract, with a license it is going to be the 100% net profit.

I was thinking to multiply 1.800 per month per 3 years, it is 64.000 with only a year of updates if any (maybe there will be not updates), if I release an update after that I will charge for it with a year update policy, can someone give me any advice?