Is the program running once every five minutes, i.e. via cron or some such, or does it run continuously and check for online data every 5 minutes? I suspect the former; if the latter the program could just keep the last price in memory.

If your database is huge and slow, there might be less overhead in opening and reading a file for the data. There are various modules for this on CPAN.

But the biggest question I have is WHY? If I understand correctly, you're proposing to store on row 2 a calculation based on field X on row 1 and 2. Why store what can easily be calculated?