I would say, realistically, a good condition series 3 with a few mods goes for high 30's. Anyone trying to sell a series 3 in the 40's is kiddin themselves (obviously dependent upon certain factors)
Bearing that in mind, my car will be soon for sale. Knowing that car prices depreciate, I know I will lose money. Although it seems that series 1's sell from as little as 26-28k for a shitbox, to 33-35k in really good nick