I have participated in Ludum Dare a couple times, though last time I was never able to think of a game I liked that was short enough to finish for the jam. However, it did inspire a couple ideas I am still working on, like my prototype for Time Harvest in Space, which I hope to finish someday.
See what you mean about not wanting super expensive things to appear too quickly. Especially for a game that essentially has one long level that does need to unfold to some extent. In that vein, maybe starting with no actions beyond Pick and having Sell and each individual upgrade be added as you pick and earn larger amounts of money in the style of unfolding games such as "A Dark Room" could be an interesting way to go.