Simulations are software applications that mimic reality in some way and allow the learner to learn by making decisions and seeing the impact of these decisions.These can vary widely in their nature.  
A mathematical simulation can display a graph that changes when the learner changes a variable in a problem.
A scientific simulation might display an animation of an experiment or phenomenon which changes when the learner changes a parameter.
Business simulators, often designed as “games”, can show how outcomes change based on the learners decisions (usually financial).
In general, simulators are time-consuming to construct and require programming and mathematical skills.  Simulations of social interaction may be based on scenarios and storyboards and can be even more complicated to develop.
So for a low-cost approach it may only be possible to use simulations if they are available as Open Educational Resources (or if you can purchase one at a reasonable cost and are not charged on the basis of the number of learners using it).