Size and quality of plant and blooms. Another factor could be rarity. If someone had roth hybrid like Woluwense or Delrosi with 6 spikes it would be worth quite a bit, or a rare albino plant, and lets not talk about CITES restricted plants.
I would also add in the time factor- and that in many cases is the most important factor of all.
You see it less now that orchids have become so mainstream and really decimated the market for breeding on a smaller scale- thus greatly reducing the number of participants and opportunity to buy rare divisions or original plants from a wider number of sources- but historically I have found that the real dollar value in super-high end divisions and original plants (and by that I mean $5,000+) is the value of being first, or at least ahead of the curve.
We still have not seen the full extent of what rothschildianum can yield. But the closer we get, and the smaller the increments of improvement, the more vital it becomes to make your sibling crosses with the most recent best plants- with the difference between those and the best plants of 10 years ago progressively shrinking making every last mm of dimension or nuance of color all the more important in striving to do better.
And given both the financial potential of flask sales and the prestige of having the current best plant on the block, it makes perfect sense that top roth divisions are traded well into the five figure range. I know a lot of breeders sell with each other because in the long run everyone needs more diversification in their own breeding lines- but for those outside that circle the greater problem than affording such divisions is finding them available in the first place when they really and truly are the current best of the best.
But once that time advantage is lost, the value goes down substantially. It certainly varies by plant and collector, but I think $5,000 is about the highest price out there for something that is a really great and historic plant but which is no longer truly cutting edge.
And then eventually, when a breeding line is tapped out- people move on and many formerly valuable plants become worthless. In the 90s, Carmela and other top Phal breeders routinely sold high end original and stem prop stud plants of pinks, whites and candy stripes for $500 to the low thousands. When Phal. Hilo Lip was all the rage- but a pain in the ass as a sib cross for all the inbreeding- the guy who flowered out a huge pink Ida Fukumura with a white lip was getting offered $10K per stem prop. You cannot even find such plants these days- the breeding lines were deemed to have been maxed out in terms of potential plus the grocery store Phals came along.
Point being- the value in the $5K+ plants is being first, or as close to first as possible, to have and breed with a plant that can double or triple the value of the flasks and seedlings sold. This premium comes either from having the best of a current species/hybrid or getting in early on a newly discovered species.
But over time that value erodes- and quickly in some cases- and so for the hobby collector it makes no sense to chase after such plants.
That said- the most expensive plants I have ever known of go in the $40-75K range. And those are original plants sold by certain high end nurseries direct to exhibitors at major international shows who want to come home with a big fat trophy. It is not unheard of for award-potential plants to go unshown to the public in anticipation of such sales.
For any of you hobby sellers- this can work in your favor on a smaller scale at local shows BTW. When I used to do orchid shows in high school and college, I would always call Carter and Holmes or one of the other big Catt houses a few weeks in advance and buy 1 or 2 really huge mericlone plants with 4-5 budded sheaths so that the plants would be in full flower for the show. Then I would set them out first on my sales table as I set up for the day, and every time I would sell them to someone needing something really fancy to add to their exhibit. The profit there was easily equal to selling 20-30 high end seedlings.