Valuation of stock vs modified RS6

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My RS6 is completely stock with 83k miles and late vin 905678. Misano red/ebony black, SE exhaust, rear side airbags. Retrofitted with RNS-E. All service receipts from every owner since new (DRC replaced at 30k, transmission output shaft seal replaced at 40k; belt/water pump service every 20-25k; oil changes every 5k) Clean title, no accidents, no lien.

I've only owned it for a year but planning to replace with a C7 S6 in the next year or so.

First, how do you come up with a price range for a stock RS6? There are so few cars for sale at any time and the mileages are all over the place. Does stock deserve a premium? And if so, which mods are lowering the price or scaring away buyers? Replacing DRC with coils, swaybars, stage 1 tune, TCU are all things I've considered.