What Flash Drives are you looking at that are that costly?
I think the original Raptors were 10k RPM as well.
SSD's don't have a problem with heat. If you mean the inside case temps are affecting the SSD, then something is seriously wrong.
The main difference between an SSD and a flash drive is a very specific controller......as far as I am concerned, the hardware might as well be interchangeable, because the controller is what gives an SSD 99% of it's performance. Nand memory these days mean very little in the grand scheme of things.
What flash drives are that costly? You haven't been shopping for the larger capacity models apparently. Have a look at one price for a 256gb I initially found priced for less elsewhere but this will show just how high they can be at times in this 2010 blog on a Kingston DT310 256gb model.
Kingston's 256GB USB Flash Drive is $1,108
The price I had been finding was actually between $500-$600 while shopping around. As for the original WD Raptors I think you will find even the 36gb and 74gb models released in 2003 were 10k rpm with a life expectancy of 1.2million hrs. for reliability.
As for heat that was operating temps when a case isn't adequately cooled. Note since they are not mechanical they won't have any internal heat built up like you would see with a mechanical but more like your average dimm would be seeing. One of the main reasons is from not drawing the power from the supply you would see from a mechanical drive as a rule there.