Sale price of limited edition items. Input requested!

[GL1TCH3D]

Hello everyone (that's reading this at least)

I'm hoping to get some help with making a more or less generalized function of the function of inputs original retail price and time since release to output estimated sale price.

Now there are obviously huge flaws in doing this since different genres of products can have much different curves.

The reason I'm doing this is basically because a lot of the limited editon products that I've followed have more or less followed a similar model of the pricing and  I wanted to see if I could make a function to determine a rough estimate for the future price of a limited edition product.

So far from what I've observed is that the manufacturer sets an MSRP.
The sale of the product starts are the MSRP but slowly starts decreasing until all the units are sold.
After the last unit is sold the price steadily increases over time.

This holds the following assumptions:
1. The item is going to stay in demand.
2. Manufactured product. Not a one off.
3. The limited edition nature would suggest that fewer units are produced than demanded.
4. The items are sold as limited edition items. (IBM Model M would not count but the limited edition breast cancer pink unicomp would be counted)
5. The item will not be made obsolete. (This is a tricky one to make but the idea is that the company won't release something better or the industry will no longer support the item in question. If the item is made obsolete the demand changes drastically to mainly collectors).


So far what I have right now is the

G(x)=(0.0146b+3.57)x+b
 
Where b is the lowest retail price that the item was sold for (generally around 25% less than the MSRP) and x is the time in months since the product was sold out by initial retailers (this doesn't include resellers)
G(x) is the selling price of the item at x months after the end of the initial sale period.


What changes would you make to the function? (or write your own!)


Thanks to Bowji. Added some assumptions to make in the respect of the model.

[bowji]

This holds the following assumptions:
1. The item is going to stay in demand.

Trying to grasp a future price of something not in production is very hard (consider the variety/price of keyboards from IBM).
I would add something to do with demand. Demand cant be a constant here since once the people who gets a hold of the product the demand will decrease unless there are more people somehow introduced to the product that is not marketed/produced at the exact same rate... Demand can increase or decrease depending on competitors reaction/trend changes/new introductions/etc. Its been a while since I learned econ but hope that helps. Knowing what the product is might bring some more input from people knowledgeable about the industry.

[SpAmRaY]

[GL1TCH3D]

This holds the following assumptions:
1. The item is going to stay in demand.

Trying to grasp a future price of something not in production is very hard (consider the variety/price of keyboards from IBM).
I would add something to do with demand. Demand cant be a constant here since once the people who gets a hold of the product the demand will decrease unless there are more people somehow introduced to the product that is not marketed/produced at the exact same rate... Demand can increase or decrease depending on competitors reaction/trend changes/new introductions/etc. Its been a while since I learned econ but hope that helps. Knowing what the product is might bring some more input from people knowledgeable about the industry.

Ah yes I should clarify. The main ones I'm considering are hobbyist items sold as limited editions. Audio technica comes out with limited edition headphones now and then. The products generally are unique from other offerings and can't be easily replicated. It should also be noted that regardless of the likeness to the regular versions, they still sell for a premium (but they also sell some that are completely unique to other offerings).

Trends changing is definitely a big factor.

The IBM keyboards weren't sold as limited edition hobbyist items so I wouldn't count those in this. I would however count something like the limited edition breast cancer awareness Unicomp keyboard that was sold.

Supply and demand would probably be the biggest factor.
If the item is truly limited in a hobbyist community that wants the product I would think that there will always be people who don't have it that want it.
The following assumption would then have to be stated: There were fewer released than demanded by the community purchasing the item.