I've seen a few game shops charge about $15-$20 just for the service of replacing a dead battery for Gen 2 games (the charge does not include the cost of the new battery, which you would also have to pay for). Having done the process myself, I definitely think the service isn't worth it, but it's popular enough that the shops feel it's worth advertising. These shops were selling patched Gen 2 games for about $35-$40; even on Ebay as well, sellers who advertise a new, working battery are selling Gen 2 games for much higher than sellers who do not.
So when the sad time comes and batteries start dying in Gen 1 games, I imagine this effect will happen again but on a larger scale. There will be a higher demand for fully functional Gen 1 games, so, taking advantage of the higher demand, the best time to sell would be right after the batteries in most people's games are dead. Of course, you'd have to replace the batteries in the games you sell first, but that procedure really isn't too terrible.