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.