I think for a SaaS, the most important aspect of validating demand is if anyone is willing to pay for your product. I didn't use Mashape (I tried, but they kept churning their product which turned me off), but that might be a good way to validate.

I ended up giving the service away for free, but officially stated that the $10/mo tier was free during beta, and if you wanted more to subscribe at $50/mo. I got enough actual revenue out of that to justify continuing.