For Electron products, the price is US$2.99 per month per device including the Particle SIM and up to 3 MB of data per month. This include Particle device cloud access (publish, subscribe, variables, functions, etc.) and the product dashboard features including fleet software management.
For individual developer Electrons (non-product), the price for the Particle SIM is US$2.99 per month for up to 3 MB of data per month. This includes Particle device cloud access including publish, subscribe, variables, functions, and OTA code flash to individual devices. There is a limit of 100 developer Electrons and 100 Particle SIM cards in a single account.
If you use an individual developer Electron with a 3rd-party SIM card, you can use the device cloud for free, for up to 100 devices per account. You only need to pay the SIM charge from your mobile carrier.
If you are using the Particle SIM card there is no reason to not create a product, since the price is the same and you get useful added features.
If you are going to have more than 100 devices, you almost certainly will need to create a product because of the developer device limit.