Hello i'am thinking of purchasing a small rv trailer what is the common cost per day to park at rv parkings ?

I traveled through 5 states and used Good Sam and other discounts and nearly all sites were under $40 except San Diego RV, which gouged. Santee Lakes is less, I think and there may be other sites that are much less. This RV place isnt that great.

Really depends on site, full hook up, partial, views, expect to pay $30 to $80 per day.

Wow how expensive, I would have tought that owning an RV was to eliminate hotel costs. Not cool at all

What is the monthly rate

Depends, you can save money on weekly/monthly rates.