What's the value of an external advisor / non-exec?

I've been doing a lot of free mentoring during the pandemic, and I've received loads of great feedback and requests to provide ongoing support.

The problem I have is knowing how best to position and price this. I need it to:

a) Be enough engagement to provide real value for the founder.
b) Priced well enough for me to justify taking time away from my existing work.

Current offering would be something like this, for a monthly fee.

1 x 1 hour phonecall, 
Email support, 
Me being a cheerleader for your business
Keeping you accountable
Introductions* 

*One of my introductions for a mentee has resulted in a trial of their software with a £120m company.

Although the price and exact structure has yet to be finalised, I would hope to bring MUCH more value back into the business than I charge. Think of it as having a low-level non-exec director for the price of a big night out.

Someone has suggested that group sessions may work, too. Say 4 people, in a group session once a month, guided by me. They get the benefit of helping each other out, too.

I've been helping startups in one way or another for about a decade, but because it's often been on a voluntary basis I'm not entirely sure about how to best position this.

Thanks in advance for any wisdom and guidance!