|I don't think that's what Ms. Sumn meant.|
In Word you can edit the auto-correct list under Tools...AutoCorrect Options.
Word uses that feature to correct commonly misspelled words, such as automatically changing acheived to achieved.
I just edited my AutoCorrect Options by adding 79.99 to be changed to 109.99. Now every time I type 79.99 it gets changed to 109.99.
Check the list under Tools...AutoCorrect options to see if you somehow added it to the list. On my machine, that entry is just under all the symbols, just above the alphabetical listing.