I've seen used MKII Standards bring as much as $300 in that condition - about what you'd pay for the MKIII Standard, new - just because it is a Mark II, instead of a III. Of course, you won't get that on a trade-in and you may have a long wait to find someone who pay that in a private sale.
As a shooter who owns six Marks/Standards, why not just keep your Standard? Targets are great, but Standards have their place, too. Quite different guns in terms of handling and feel. As much as I love my Target versions, I shoot my Standards quite a bit - my Standards get the nod for plinking and carrying, out in the woods.