How do you run a risk / gambling experiment using Qualtrics or Amazon Turk?
This would be easy to implement in Inquisit. Inquisit has a web interface and is particularly designed for branching and randomisation of trials. If the coding sounds too complex, you can always contact Inquisit and pay them to program the task. From my experience, the cost is not that great for simple scripts.

You can see an example of the Iowa Gambling Task in Inquisit here. That's a lot more complex than what you are describing, but it should give you a feel for what is possible with Inquisit.

You could set up Inquisit with Mechanical Turk as you would most external surveys. See this discussion of mechanical turk.