Well, seeing the cellphone companies make their pricing as obscure and confusing as possible, refuse to tell you how much the taxes and fees are, and do everything they can to avoid showing you their cheaper plans: good luck with that. (Same with ISPs.)
The basic cost is typically $10 to add an iPad to a shared data plan with your smartphone. I pay $10.47 including the taxes (Verizon, in Alameda, California).
I assume however you are trying to research standalone plans, so cannot help, sorry. Would be interested what you find out.
I assume you know that cellular is not really needed for an iPad if you always have your smartphone with you - just use it as a hotspot (at the expense of phone battery usage, though).