Depends on the frag, the size, the rarity and or desirability. Also the sellers motivation factors in. Some people fire sale corals when getting out, others give stable, good sized frags for more.
What do you have?
If you want to pm me I could tell you what I'd ask.
To answer your original question, no direct index, but plenty of websites with frags for sale as a reference point. But remember there a lot of hobbiests in the area and common corals might fetch lower than market price.
Hope this helps.