How can we assume Brightbox won't raise prices?

How can we assume Amazon won't raise base prices, possibly affecting the spot market, then again this is a spot market! It's like asking how can you assume stock prices (not exactly equiv, but thats not the point) will go up or down?

How can we assume anything?

We don't. I think the assumption on the wrong feature to market is the problem here.

I only care that right now, EC2 spot are cheaper by a large margin. IF the price raises to match then I'll adjust. I'm not locked in.