The simplest method depends on what equipment you have available. If you are trying to use a kit, it will cost more per test than if you set it up yourself with the same reagents purchased separately. Minimizing the assay volume by using microplates also decreases the cost. With a 384-well plate, you can do a test in 30 µl or less, if you have access to a 384-well plate reader.
If you want to use a kit, maybe one of these is cheaper than the Amplex Red-based ones: