Great looking Prologue!
As far as value, sorry, but I have two Prologue road bikes myself, a 1987 and a 1988. Both are in pristine condition. IMHO this is not a $685 bike, or anything close to that high. If you can get that kind of money for that bike, take it quick!
I saw your note about the upcoming move. If you can get this kind of price where you live, take it. Unless you are moving to some red hot bicycle market, you are not going to see this kind of price elsewhere. There was a local in my area trying to sell a FG Frankenbike, that he bought in Portland, OR for $600. I watched week after week as this seller started to sober up on local values. Bike dropped about $50 per week. I remember seeing it finally for under $250.
As far as the Sante comment, that is totally bogus. The Prologue was not sold as a complete bike, it was sold as a frameset. So it did not come with Sante, or any other component group for that matter. The buyer would then have it built up by his local shop with whatever group he wanted. I have had mine built up with Shimano 600 tricolor, then full Sante. Right now, mine is nine speed Ultegra 6500 STI (OK, someone with a sharp eye will pick out the Tiagra hollowtech compact crankset). My second one came with Shimano 105 group. The Shimano 105 parts were moved to another bike.
So back in 1987, if you wanted Sante, you just paid your shop for the Sante bits and had it built.
By the way, your bike is not full Sante. It is missing one of the sweetest (IMO) pieces of Sante, the Sante headset. Those also are Shimano 600 pedals (which are my favorite). The headset on your bike looks like a Tange Falcon, which did come with the frameset from the factory.
There is no catalog page for the 1987 Prologue. Apparently, the model came out after the catalog was printed. You will find it in the 1988 catalog, where it does mention the frameset being in its "second" year.
I looked for two years to find my first Prologue, ended up getting it on ebay from Utah. I have since found several (go figure), including a couple local.