Find 5 bikes (or more) for sale in the same condition and miles, add it up divide by 5, thats what they'll pay... or nada which ever is higher.
Always wait for their first offer though, it may be higher.
I've done it this way more than once with bikes and cars, worked every time with no hassles.
I once had a 72 super bettle and it burnt to the ground. I figured it was worth around $2000. Insurance guy offered me $3800.