Here's what I found for a 24" ball valve price online:
Size 24 inch (DN600) Pressure Rating Class: 150-1500 (PN20-260) Operation: Gear Operated Unit Price: $6900-$32000
A couple things to note. These valves are manually operated. Inconel is much more expensive than 316 stainless steel. Inconel is much more difficult to machine than stainless steel. These differences would likely add significantly to the final price.
Found one reference which states that Inconel machined pieces would cost about 5 times more than 316 SS machined pieces.
On average, inconel pieces cost up to five times more than its stainless steel counterparts because it is a much rarer material and requires more work to produce as a result.