I'm just saying it's theoretically possible that a specific Regent cruise could come out less expensive than a similar Seabourn cruise if you included Seabourn air, hotel, & transfers and a Seabourn excursion every day and the Seabourn fare was not on sale.
I would think the differences could vary a lot depending on the specific itinerary & dates and the assumptions you make about air, hotel, & excursions. Fare sales make the comparisons even harder. So, I don’t think it’s possible to make a general statement. I have never done a price comparison.
I’m also saying I would pay a significant premium for Seabourn because of my experience with food & service on both lines.
I do not have any inside information on the prices the cruise lines pay for hotels & airfares. I would expect that they buy their air tickets through consolidators & I am sure they pay less than the rack rate for hotel rooms purchased in bulk. I don’t know how much Regent increased its fares to pay for “free” excursions, hotels, & air or how much Seabourn marks up their excursions, air, & hotels.