$1,800.00 to $3,000.00 (minty) depending on bore condition. If it's a 75 it's likely to have had a few rounds through it. If lands/grooves aren't in great shape you could be looking for as little as $800.00
$2000 is probably pretty close. Thats just for the gun too, without "extras".
HK accessories are usually worth their weight in gold too, so a port buffer, claw mount, bipod, carry handle, extra stocks, etc, can quickly increase the price.