Promo kits are recent ( past 15 years or so ) attempts from the manufacture to get more mileage out of the tooling.
simply put the un-assembled promo in a different box with a different label,,,,,, whalla it now caters to the model builders crowd as opposed to the promo collector community.
As mentioned some promos will barely bring $5 - $10 range, these are either seriously damaged older original promos but most are recent issues ( think mid 90's and newer ) simply because the market is flooded with them .
SOME promos will bring some serious coin, a 1969 Dodge Coronet R/T convertible promo as an example.
I collect Mopar promos from the 60's thru early 70s time frame and even the above is WAAAAAAY out of my reach