Negative HCG Test

I was just curious what a negative HCG test read? I had pills inserted in me 11/12 after I found out I was having a missed miscarriage. I had blood drawn just before the pills were inserted to test my HCG level. It read 75,000 on Tuesday. I went back on Fri 11/15 and the HCG level went down to 9000. I had blood drawn again yesterday and my level went down to 683. Is that a normal speed for those levels to decrease? And what level is considered a negative? 0?