It's not just the total price, and it's not just the risk that you may disappear, it's also that the client will likely get a different product under the two pricing structures. In the flat-fee case, objectives are defined solely by the needs of the client; in the SaaS model, they're split between what the client needs and what you want for your future product. The split incentives can go bad for the client in all kinds of ways:<p>- they get a product that is kind of what they wanted but not really, because you built it to cover what you see as the general use case (i.e., the one that you might be able to sell widely), while they wanted you to cover their specific needs.<p>- alternately, they insist that it cover their specific needs (they started this process after all), and you end up fighting with them over design and over who should cover the costs of customization.<p>- maybe you were lucky and there weren't any of these conflicts with the first round of development, but now they want a second round that will take the product in a direction that isn't consistent with your plans for it. What then? Do you go to a hybrid pricing model, where the monthly fee covers the "base" product and they have to pay a one-time fee for their extensions? Do you tell them to take a hike and start all over again with a new dev, because you're not interested in their unmarketable extensions?<p>As others have said, we charge on T&M and make sure first and foremost that our client is happy. AND we retain a right to the code. If we think there might be a broader use for the tool, then AFTER we've made the client happy, we'll branch and adapt it to our sense of what is marketable.