We describe Cümülön, a system aimed at helping users develop and deploy matrix-based data analysis programs in a public cloud. A key feature of Cümülön is its end-to-end support for the so-called spot instances ---machines whose market price fluctuates over time but is usually much lower than the regular fixed price. A user sets a bid price when acquiring spot instances, and loses them as soon as the market price exceeds the bid price. While spot instances can potentially save cost, they are difficult to use effectively, and run the risk of not finishing work while costing more. Cümülön provides a highly elastic computation and storage engine on top of spot instances, and offers automatic cost-based optimization of execution, deployment, and bidding strategies. Cümülön further quantifies how the uncertainty in the market price translates into the cost uncertainty of its recommendations, and allows users to specify their risk tolerance as an optimization constraint.