2001 Ford Taurus RPM not working

2001 Ford Taurus

My RPM gauge hasnt been working for a very, very long time. Sometimes it will work briefly (idk if its accurate or not). I called the dealership asking about it but they told me it is either the cluster, which im not sure if it is the culprit, or a circuit problem and they wanted me to bring it in to do a diagnostic test for 100 dollars, screw that idea, im asking for any opinions that will help, I can get the cluster replaced brandnew off of ebay way- wayyy cheaper than their cluster price of $650.00 dollars, yeah I know, just for the cluster.