The H&K 91 A2 at 80% (some minor wear showing) is worth about $1600. However, because of market demand, you will find the pricing to be in the $1850-$2150 range. If you found this rifle at your local dealer, expect another 15-25% mark-up. If you found this specimen at your local gun show on a private table, you would pay maybe $1900.
Unless this piece is fitted with all factory accessories including scope mount, bayonet, bipod, sling, steel magazines (at least 4), and has the original red manual, you shouldn't be anywhere near $2500. You'll see prices on these without the accessories in the range of $2400-$2800, but they rarely sell in that range, unless they are 100% and you have a serious and wealthy (or naive) collector interested in it.
They are great rifles, but you need to shop around for a better price/condition combo. Resale values tend to strictly follow condition. The closer the condition to 100%, the higher the price tag. At $2500, this rifle should be new in the box.
The AR is also over priced. But it is the sense of urgency that people have in obtaining these things that drives the prices up. Again, condition is everything for resale value. Look for one in as-new or new in box condition, and that will justify the higher price.