I am also developing a variant of the Nova, so I have been investigating certification recently. First, a device is not awarded a certification, a manufacturer is awarded the certification. This means that even if you want to manufacture an exact duplicate of the Nova, using exactly the same antenna, the Hologram certification does not cover your device. You will need to arrange for your device to be separately certified.
Each network provider has its own certification requirements, so if you want to an ATT cert. you need to go through their process. If you want a Verizon cert., that’s a separate process, etc.
Price is a very good question and there simply is no simple answer. You could try to get yourself through the certification process for a particular carrier, but (imho) you would be foolish to think that without deep expertise and long experience that you are going to be able to navigate the process without making numerous mistakes (which will cost time and money) which an experienced guide would have helped you avoid.
The price for such guides is all over the place. Some will give you no design advice because they fear you’ll fail and then hold them responsible because they advised you. Others will provide great, practical advice for a fee. For example, if you decide to an antenna into your pcb (which brings the production cost for the antenna to pennies), some of the guides will happily do the antenna design for you. If you undertake to do this yourself, and you are not a genuine expert, you will likely get a terrible antenna.
I have been considering certification for Verizon only. It is my impression, based only on my own experience, that Verizon’s LTE network is available most everywhere that ATT’s is available, but not vice versa. I posted a question on this form a month or two ago asking what is the experience of others on this matter, but that thread died a very lonely death.
If you want to discuss the features you are seeking to add, maybe your new features and ours will align and we can work together in some way–or not.