I think you need to be thinking more along the lines of flow requirements.
An example above used an existing 1.5hp pump. If that is what the pool and plumbing were designed for and you put an Intelliflow in there and ran it at a very low speed with the comparable flow of a 1/2hp pump for instance there of course would be a huge cost saving. But if you ran that pump at the same flow as the original 1.5hp pump there would not be. If you replaced that 1.5hp pump with a 1/2hp pump there would also be significant cost savings. If it was a sand filter however you might still need that higher flow rate to properly backwash. In this case a 2-speed pump might be the best choice. Really only 2 different flows are required. Also you will have to make sure that the low speed flow is enough on that pool to keep it properly circulated and clean.
If you had a pool spa combo where the pump is also expected to perform as a jet pump you might have a good candidate for a variable speed pump. Imagine giving the client complete control over the power of the jets. Add one or more waterfalls and/or spray features etc and you now have a prime candidate for variable speed.
I don't think you are going to realize any cost savings by putting a variable or multi speed pump in place of a larger hp pump and running it on low speed all the time on a cartridge filter pool over simply installing a smaller pump.
Of course you can't beat the Intelliflow with its safety features.