Business and pricing are not my areas of expertise, so feel free to ignore or improve on this idea. My suggestion is to use Flickr to upsell Portfolio and Pro SmugMug plans (maybe with a nominal discount of $1/mo for Portfolio and $2/mo for Pro). Flickr could be the place where photographers get feedback on their photos and improve their skills, and then SmugMug is where they build a more curated portfolio of photos to sell based on what resonates with people on Flickr. When a photographer has reached a tipping point of positive feedback on Flickr, they may feel ready to start selling their photos, and that's your opportunity to upsell an appropriate SmugMug plan.

Or if you're confident that you can identify photographers whose photos will sell based on engagement with their photos on Flickr, offer an introductory rate for the first year so they can test how well their photos sell. If they sell enough photos to cover the cost of an upgraded plan, paying for an ongoing plan is a no-brainer.

At least, that's my use case as a hobbyist. I currently pay for a basic SmugMug plan. I daydream of upgrading to a Portfolio plan to sell some photos (just for fun), but I don't know that I would manage to sell any. I'm now thinking Flickr might be a better place for me to start testing the water.

As an aside, thank you for removing the Yahoo login. That's been more of a barrier than you would expect for me every time I've wanted to use Flickr in the past. I don't use Yahoo for anything other than Flickr. I think I have more than one account, but I'm not sure, and I don't remember which account has my Flickr albums. It's just a mess. I'll definitely be giving Flickr another chance in January.