Presently IBM's just getting in the space. I was offered a chance to use BlueMix during the closed beta. I didn't for the same reason you mentioned: they didn't have a pricing plan. I too thought, "What's the point of building something if it will be really expensive later?" Fortunately, IBM seems to be reasonable when they came out with pricing.
I wouldn't build my company around the beta of Watson, but if you experiment during beta, you could have first mover advantage when the pricing comes out and you find it acceptable.
While it's not something that you'll have running completely on your mobile device, it's something that you can easily run on the backend server. Fortunately, BlueMix provides lots of very useful services for mobile devices, including automatic crash monitoring, user feedback, and cross platform push services. Feel free to find me on twitter (same name) and I can provide more details.
I dont think they were intending for this to go out on production applications just yet. You could just play around with the API in some personal or open source project for now. Whatever pricing plan they come out with should be reasonable.
Like - I'm not really going to be using this in my next Android / IPhone app any time soon.