I don't think you can pick and choose the plans per user? We're currently using the security features of GitLab Ultimate and as far as I've understood it, that's $99/month/user for every user you need on your GitLab license.
I think enterprise software agreements can often be negotiated.
In the case of GitLab, their pricing page has the usual easy ordering buttons for each pricing tier, but the $99 tier button also has a prominent "Contact Sales" link right below the button. https://about.gitlab.com/pricing/
Once you're already paying sticker price, and have invested in using it (i.e., moving to a competitor is a lot of work), your negotiating position will be different, but the vendor can still lose the account, if customer is unhappy enough to eat the moving costs.
In the case of GitLab, their pricing page has the usual easy ordering buttons for each pricing tier, but the $99 tier button also has a prominent "Contact Sales" link right below the button. https://about.gitlab.com/pricing/
Once you're already paying sticker price, and have invested in using it (i.e., moving to a competitor is a lot of work), your negotiating position will be different, but the vendor can still lose the account, if customer is unhappy enough to eat the moving costs.