Perform Math Function on Cell With Text and Numbers

There are a couple of options, but the big question is what do you want it to do?

You say you want to 'multiply the contents of this cell', but "<100" doesn't translate into a number that you can multiply by, so what should be used in this case?

Also, how generic or specific do you need to be? Since you say you're using a pop-up, that implies there is a finite set of values, and only one(?) of these causes this problem (the others are numeric values?) Are there other values? Are these values likely to change over time?

Given what you say, I'm assuming there are a series of numeric options and one that says "<100". You want to take the value and multiply it my some other value. Given that case, the simplest solution would be to use IFERROR()

=IFERROR(B2×A2,A2)

IFERROR() calculates the result of the first argument (in this case, B2 x A2). If that checks out (doesn't throw an error) then that's the result you get. If it does throw an error (because B2 contains text, not a number) then it returns the second argument (in this case, whatever's in A2).

Now whenever you select a numeric value from the popup menu, you'll get that number multipled by A2, otherwise you'll just get A2.

This formula will work for any popup option that throws an error, however, all errors will do the same thing. If you want different results based on different pop-up options (e.g. "<100" or ">1000") then the formula will need to be extended, which is why I asked about the setup earlier on.