2003 Ford Focus Speedometer

My boyfriend and I are about to buy this 2003 Ford Focus ZX5. Apparently the speedometer does this thing where sometimes it works just fine, and other times it will act normal up to about 60…then the needle on the speedometer just starts climbing (even if you are staying at 60) it goes up to like 80…and just sticks there. Then when you slow down, or stop, the needle just falls all the way to the right…and no longer works. How big of a problem is this? Is it easy to fix?

It requires replacement of the cluster. This is not exactly cheap.