In all but the most extreme situations, figuring electricity will cost less than $5.00 per day is safe, usually it is much less than that. In many states, charging for electricity is regulated by the state. Usually, parks cannot charge a rate higher than the rate they are being charged by their provider. To do so would make them a utility provider and that is prohibited. Parks in those states are often allowed to charge a meter reading fee, but that is also state regulated. Be aware that the rate the RV Park is being charged is likely higher than the residential rate in the area.
The overwhelming majority of short term stay parks just include electricity in their daily and weekly rates. Yes, there are a few parks that try to manage what you can and cannot operate, but they are very few and very far between. That being said, many will restrict your connections to the pedestal at your site, preventing someone from running extension cords to neighboring sites and running 8 space heaters at a time. They may tell you that running your air conditioner with the windows and doors wide open is unacceptable. And they may have a few other rules designed to keep waste to a minimum since common sense is not always common.