Based on my experience being in sales (talking with salesman and managers, reading trade mags, etc.), as well as my personal experience selling technology extended warranties (a stain on my soul, and the worst task I was required to perform to keep a retail sales job):
Most of the time, about 40-80% of a 3rd party warranty's consumer-facing cost is paid to the company doing the selling as opposed to servicing the warranty (the "service" for most companies involved in this business is avoiding having to provide a service at all). The amount paid to the worker trying to convince you to make the purchase ranges from nearly $0 (for non-commissioned employees), to 10-50% of the company's cut.
In cars, my salesman acquaintance told me that for an extended warranty the spiff (incentive provided to whoever could make the sale) would be $500-$1200, depending on the cost of the warranty and if you were working for new or used cars. He complained that sometimes the sale of the warranty was more important than the sale of the whole car, because the profit on the warranty was higher both for the dealership and for his own paycheck! He told me of managers giving sales speeches that said if you don't sell a warranty, its hardly even worth selling the car, and of back-office games where sales managers offered discounts on the sales price to cover the cost of a warranty because then the manager got a bigger commission and the salesman got less. Sometimes they would run competitions that was based only on warranty closing percentage, not on amount of cars sold (of course if you didn't sell enough cars per month you would be fired).
In technology warranties, the world is remarkably consistent. In some companies, there is no spiff - no commission. In places that have it, a sale of a warranty costing ~$250 to the customer had a margin of 0.5 (cost to the company selling it of $125), and would give the employee registering the sale a payment of $20-$40 (the company liked to sometimes run double-spiff competition periods, and changed the totals every few months it seemed). The percentages per warranty were not consistent percentages - I was never sure why.
As for all the people trying to make you buy something you don't want, based on my old sales training being asked to make the sale 5 times would be considered a medium-low level of sales pressure. Some of the old sales training manuals insisted that if you weren't told "no" 7 times, you should not accept no as the final answer.
My old retail company often circulated memos to us pointing out that warranties were the highest margin (most profitable) item for sale in our entire store. If you did not ask every customer if they wanted one, for every single eligible item (once while you helped them on the floor, and again at the register - even if they said no earlier), you would be told this was a fire-able offense (most managers preferred a 3-strikes written warning system), and it was the minimum expected of you as an employee. Dedicated sales associates were expected to ask more, it was suggested you recommend them personally as a good deal, and "success stories" were circulated regularly - and occasionally just made-up.
Finally, in terms of consistency, the main consistency is just the same sort of market forces that use to suggest 100% gross markup for retail items as a rule of thumb (this was before high-volume low-margin stores ate so much of the retail market). As the companies selling these sorts of things have had their margins squeezed, they look for higher margin items to grow their profits. Too high a margin and it would be constrained by other competitive forces, too low and they aren't worth pushing on people.
