Bruce,
If its got a 50 sticker on it then you don't need to bother chjecking the pickups, as its an ST-50, which means it originally sold for 50,000 yen.
Take a look on eBay to get an idea about price, but in the end I guess its worth what someone is willing to pay for it.
In the UK I haven't seen too many Springys up for sale recently. Goldstar sounds (what the Springy became around 84) tend to go for between 250 and 400 GBP depending on condition, so its going to be worth more than that here.
Peter, whats up with the serial number? Must be original if Bruce has had it from new. Theres an 81 ST-70 here that has a similar number:
http://brickvox.virtualave.net/mygtr/index2.html
James