Let me have a think about it and I’ll get back to you.
It’s a polite way of a potential client walking away without saying no directly.
So in this situation you do one of two things. Accept the no and move on. Or dig a bit deeper to understand why the no.
Often it’s because the person just isn’t fully convinced. Depending on how bold you’d like to be you could ask a question like this.
Often when people say they need to think about it’s because I didn’t explain something clearly enough or you have a question I didn’t answer. Could you let me know if there’s something I missed?
Then pause and wait for a reply. If something does come up address it and you may well get the client. At least you will have tried.