There is a lot to unpack and consider in the OP. First, the data points:
* US regional site
---120,000 unique visitors/week
---aka 120K * 52wk/yr / 12 mon/yr = 520,000 unique visitors/month.
---5-million page views/month
---aka 5-million pv/mon / 520,000 visitors = 9.6 pv per visitor.
* 54 other US regional sites
---15-million page views/month
---aka 15-million pv/mon / 9.6 pv per visitor = 1.5 million unique visitors/month.
Note: extrapolated from main site stats as likely max. May well be less undercounting unique visitors.
By these numbers you are looking at between 6.5 and perhaps 8-million unique visitors per month. On the face of it well worth selling direct ad space. However, several points:
1. are all the sites within the same niche or at least within a single overarching niche?
If yes, then they can be marketed together to advertisers.
If no, then they need to be appropriately segmented then each grouping marketed separately.
2. what is your level of confidence in the humanity of those numbers?
If you are using common hosting solutions and relying upon GoAn to identify bots I'd suggest you are undercounting bots by 50%. Conservatively.
If you are using Cloudfare or similar hosting I'd suggest you are undercounting bots by 20-25%.
Further, if those uniques include 'good' bots they need still be removed from calculation.
3. what is your current Google banner ad conversion rate? MoM? YoY? Velocity? Trajectory?
This is the foundation from which all conversion calculations will be derived until you have explicit direct ad comparisons. When selling to advertisers you need hard numbers based on existing reality. And it is usually better to under promise.
4. what is the source of those traffic numbers?
Is it mostly/all Google search referral? Or is it more diverse?
If diverse, what is the volume and CR for each referrer? Second, the value on offer:
This is a serious problem. You are not going to outcompete Walmart nor third party ads networks with a similar offering. 250x250 (or your 300x100) ads were 'the' way to sell direct space 15-25 years ago. Once AdSense et al arrived they had, by a decade ago, basically wiped that format from viable contention.
Ye olde typical text/image ad offering is unlikely to provide sufficient value above similar third party offerings. You have to offer uniqueness, customisation, differentiation, etc. The most popular methods the past few years have been native ads, in image/vid product placement, in content mentions/testimonials. However, there are others and variations and combinations.
The critical points are:
* the offering must be sufficiently 'other' to tempt the advertiser to trial change.
* it must be dramatically 'more' to garner the 10-20+ times the usual third party network rates.
* imagination is almost a requirement.
* do NOT set pricing too low. If you do not value your offering why should the advertiser?
* unless you are targeting small business you need to network and get introductions to CMOs and ad agency product managers. These folks aren't interested in sales type people. They will require professional presentations.
* small business will nickel and dime you to death so set a level for an offering and be willing to walk; think advertising NOT listing service!
* learn your niche advertising; it's players, their budgets, their venues. And price accordingly.
For idea reading, some threads I started, a decade ago, at Cre8 (RIP), courtesy of IA WayBackMachine:
* Lift Up Your Eyes From The Adsense Plains, There is Ad-Gold in Those Hills Over Yonder
[web.archive.org], January 2008.
* Lift Up Your Eyes II, Ad-Gold Has Been Found in Web Multimedia
[web.archive.org], July 2008.
* A Webdevs Dream: Ads-as-content, Coming Up From An Underground Near You
[web.archive.org], September 2008.
All that was in play a decade ago by a few sites. It's almost mainstream now. Most webdevs are wrong in what they think advertising on the web is or where it is going. You need to get caught up, the past decade has been wild and the next is looking wilder. :)
But my issue is a little different. I get over 1,000 emails a day (not counting spam) that need replies, I have to moderate the message boards and classifieds, I handle all of the web design and development, as well as networking... there's just not enough time in the day. I literally wake up and start working, then work until I go to bed.
Web design is a priority because I desperately need to rebuild my sites to be more modern and mobile friendly, but I've seriously been working on that for over a year and I'm not even halfway done... because I have so many other duties that I'm juggling at the same time.
I don't make enough money to hire someone to do any of those jobs while I do the ad sales. It would require at least 3 or 4 full time employees, 1 or 2 of which would be skilled labor that would expect a relatively high salary.
So instead, all that I can afford to hire is a commission salesman. But that seems to be a dead art.
It's a catch-22 that's seriously killing my business. I can't do everything, and anything that I put on pause to do something else ends up hurting the business as a whole.
Once you fall behind it takes all you can do to not fall further and further...
Unfortunately, direct ad sales is NOT a typical B2C sales position. It is B2B. And it requires a solid understanding of your niche, site, business; the advertisers, ad agencies, and the principle players in same; etc. Because of existing contracts and budgets it may well be one to two years from agreement in principle to contract for larger organisations. Smaller businesses can change quicker but are less inclined to change.
Finding the right ONE person, if you are unable to do it yourself, is critical. Perhaps a webdev who can free you up to handle it. At least you understand the need.