Bear in mind, there are two different models. One is the R6520 and the other is the AR6520. There are some differences between them besides the AR6520 being marked "Restricted for Military/Government/Law Enforcement/Export use only". The AR6520 was produced post-ban. Here is my AR6520 with all of its goodies. Attached File
As far as the R6520, some were produced with sear block pins and some had the lowers with a block machined into them. Earlier R6520s also had .170" hammer/trigger pins and half-moon bolt carriers as well as a couple other differences. Many of the differences are dependent on when the R6520 was made.
Value is going to be dependent on a few factors: year of manufacturing, condition, and market. Right now, the market is relatively good for R6520s as many, like myself, are wanting models that are no longer made and are gaining in collectible status. R6520s and AR6520s (like mine) are commanding fairly high prices these days. They can range anywhere between $1500-$2400 dependent on condition and whether they have their original boxes and accessories. There was a time where you could buy PD trade-in R6520s for around $600-$700 that were in fairly good condition. I was lucky to have acquired my excellent condition AR6520 for a really good price ($1200).
I hope this gives you a rough idea as to what your 6520 may be worth.