Source: used to work in corporate event technology
This assumes the conference throws out the TVs at the end. The rental company is supposed to be able to amortize these costs over years of renting the same TV. Even at the 10% breakage rate, you'd expect to rent a TV 9 times before it breaks on the 10th rental. This should allow you to rent a TV for closer to $50 plus labor and a little margin for profit. That's why renting is usually much cheaper than buying and not slightly cheaper than buying 2.
Your idea about the conference buying spares for everyone is interesting. Unfortunately for one, the financial incentives are just not setup in that way. Generally each booth is responsible for its own gear. Second, everyone needs different sizes, resolutions, mounts, color profiles, refresh rates, input types, network control options, blackout behavior, etc, etc.
Also, the discount vs extra expense of the gear is correlated to the cost of the gear. A $100,000 projector (mid-range) will obviously costly less to rent for three days, and is well worth it. With a TV though, they rental company is able to charge a premium because their customers simply will not purchase that type of equipment for all of the reasons above.
The marginal equipment cost for providing a TV is the cost of the TV divided the number of events you expect it to last through. This doesn't change if you have spares. To get close to $900 for a $500 TV, you either need extremely high overhead and labor costs or TVs that break a little more than halfway through the rental. If a TV can generally survive 30 days of use, equipment costs only justify $50 of the bill for a 3-day rental. I don't see how you would get anywhere near $900 from there.
Of course, at a (US) trade show, if your rented $900 tv breaks, even if they have a spare one, they will take a day to find a cart (for which you have to pay rent) to ride it to your booth (which has to be done by two union members, can’t be done while the trade show is open, so has to be done after 5 PM, doubling labor costs and requiring you to pay for keeping the floor open, too (even though they can charge that to almost all exhibitors)), and you have to pay for union members to plug in that new tv. All that hopefully after other union members unplugged the old tv, or you will be charged for using two power plugs.
(I have limited experience with this, but from that experience, the above seems only slightly exaggerated. On the trade shows I worked on, you weren’t allowed to carry the booth material you carried all the way from the airport to your booth)