Sorry to complicate things but, if I've understood your question, what has been said isn't entirely correct. I may be wrong, but here's my answer.
StillOptimistic, you are talking about the offered 'upgrade' price when you do a dummy booking but don't actually buy?
This is usually (always?) just the difference between the fare you have selected and the cheapest available fare in the higher cabins on your chosen flights. If you price up the higher cabins on your flight separately, you will find that the total amount is the same.
It is not an upgrade at all, really -- it is just trying to entice you to purchase a fare in a more expensive cabin to start with!
Is the price always the same? No. It varies according to what is available on the particular flights that you have done the dummy booking for. So if you do a dummy booking for economy and it just so happens that the upgrade price to UC is, say, £2000, you may find that other flights on the same day have cheaper 'upgrades' because cheaper seats are still available in UC on them. This is one of the pitfalls of using these 'upgrade' fares -- you only see the price differentials for the flight you have selected, and that may not be the cheapest higher-class flight on your chosen date. That can make a colossal difference with an UC fare!
Is the option always there? No -- it varies and I assume (like northernhenry) that loads are the principal determinant.
I disagree that the chances that the price will come down are slim in the extreme, though -- it just depends on the straight fare differential. Sales can and do affect this. I have been watching the differential between economy and UC on a particular flight in May for several months now. Each time the VS UC sales have ended, it has rocketed. Each time another sale has been announced, it has gone down to around £1,000. When the economy sale ended, it narrowed again. And so on. BA offer a similar facility and at one point the differential between economy and premium economy suddenly dropped to £14 for a particular flight!
So my advice would be to search for the cabin you want on the day you want. If an upgrade pops up that looks very good value, great, but be aware that you are probably paying the same as you would pay if you had searched for the higher cabin to start with.
I hope this makes some kind of sense.