I have a nice standing case for an 1800 singer sewing machine, but the sewing machine has been removed before I was the owner.

What would be the value of this?

Look on ebay.com if you are in the US, or any number of other ebays around the world.
Value of a hard to ship item depends quite a bit on local demand. Sometimes craigslist (with a picture) or your local paper is best. Or just ask antique stores and consignment shops. Since condition is important too, you can't just say the brand and year - pictures!

(You say 1800 - you must mean 1800s. Singer didn't exist until over 50 years later. I'm going to guess here that without the machine it really isn't worth much, unless you are super lucky and someone is looking for one to match their machine.)