In not entirely sure. Regular top links of today range from $20-$40 depending on what you get and whose selling it. Since that came off a classic Deere, the price would be a little more than the modern ones. i would maybe check eBay or google search it, results on prices may be better. i an just giving an educated guess.
I have a nice 630 with original top link. A friend told me once, don't leave your tractor set out so someone can steal that link, it'll cost you at least 200 bucks to replace with a good one. I prefer an aftermarket one myself.