Ok. Here's what I've come up with. It's a pretty simplistic solution, but it may well work. I've assumed that Cell B1 shows the target price and cell C1 shows the current price.
In cell A1 on the worksheet, I entered the following:
Now, in the ThisWorkbork section of the VBA window, enter the following:
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Excel.Range)
If ActiveSheet.Range("A1") = "YES" Then
MsgBox "IBM Has met its target price of $90"
The only problem is that the message appears every time the price changes when the formula in A1 is valid.
I hope this helps you a little bit, or at least gives you some ideas!