Your phrasing is quite idiomatic and fairly neutral, so it might be that your customer simply doesn't want to pay that much! You could try adding a phrase that shows you would consider a different price. How about:
"I am looking for round about the $150 mark."
This shows you would accept a price in the region of $150 dollars and that you are prepared to be flexible, so maybe they would feel more confident writing back to offer perhaps $140 dollars?