Dave
There is really no formula to set the price. This is a used product and there are several generations of newer keyboards already out. This model will not have a great demand. The price is only based on what the market is willing to pay for it.
My recommendation is to first decide if you want to sell it at whatever you can get. If the answer is yes. Then, first, get an estimate on the shipping. You will need the weight including the packaging. estimate a distance far from you (from example if you are in LA, estimate sending to NY). Add the shipping cost to the price and advertise with 'free shipping" otherwise people will find it less attractive (most of the music stores send the stuff with free shipping).
I would advertise on eBay as a 'buy it now' Advertise on eBay for $800-$900 and then reduce it by $100 from time to time, every week or every two weeks. Also, take into account the fees from eBay and Paypal as well as the shipping cost. Your net will be reduced by those fees.
I think that with this type of product your mindset should be that whatever $$ you get you are lucky.
I have sold on eBay a couple of times. I sold a Tyros 4 and a Tyros 5 as well as some Yamaha monitors. The first time i was extremely disappointed, mostly because my expectations were high. I was thinking that people would pay me what i thought would be a fair price for items in mint condition. Also, because we are attached to our keyboard we tend to include our sentiments in the price.
There is no such a thing as a fair price. People will offer you whatever they are willing to pay regardless of what you may think or feel.
Realistically, i do not think you will get $900. I would think that if you get a net $500 out of it, it will be a good sale.
You can also advertise it on this forum. . . . however, your chances are probably greater on eBay just because it reaches more potential buyers. . . .
Hope you sell it. (or keep it if you are not willing to go thru the aggravation).
Good luck.