First of all: $1000 as a raw literal value means 1000 in hexadecimal or 4096 in regular people numbers, not to be confused with 1000 dollars. Now that that's out of the way...
Looking at the original code, I can find exactly one instance where the jackpot is set to 1000 (dollars, that is) and that's when you win whatever the jackpot is at the time. You win it, you get so many silver dollars, and the jackpot resets to 1000. Which tells me that it doesn't actually start at 1000 dollars at all, not by any script's doing.
But looking at Sluicebox's decompilation, there's an extra bit in script 155, the password room, in the LoadPassword function. In the original source, it tries to open the password file, and if that succeeds it reads the password and jackpot value. But in Sluicebox's decompilation and mine alike, there's an else: if it failed to open MEMORY.DRV, it defaults to "NONE" and 1000 dollars.
Another funny part is that ScummVM, for intricate technical reasons involving some of the platforms it can run on being overbearing about file security, does not necessarily use the same files in the same position, both for gimmicks like the above and saved games.