its a pretty standard model i see nowadays - pay an upfront fee for the software, then an annual maintenance & support cost thats usually a smaller percentage of the original cost itself
a lot of companies are doing this now, especially online software - just look at adobe themselves, they went wholly to a subscription model!
if the client is paying for development then it might be hard to sell the above models, however theres nothing saying that if they want you to continue maintaining and supporting it after completion that they need to pay an annual or monthly fee - i've seen that too - remember time is money and there shouldnt be an expectation for you to support a product forever for free when they paid for the original work (unless you explicitly license it as such - eg selling to public and offering free updates)
