@Aaron – I agree they will spike early before falling off. The spike will come when Wizards announces Modern and then again when they announce the reprinting. The question is whether the spike will be much higher than the current one, since I think it's pretty clear there is going to be a short-term leveling off of prices. Modern hype has dominated the last two weeks, but hype out of US Nats will dominate the next, and Shocks are going to cool off. It all depends when you bought in. If you got in early, then selling now and not taking chances is pretty much what you want to do. If you bought in later, you need to wait for these expected (but not guaranteed) spikes. Thanks for the feedback, you raised some good points.

@Chas – Thanks! Fetchlands rotating is one of the major factors for me in this decision, but there are plenty of options for cycles outside of Shocks, so it could go either way. We also know the rumor that the set after Innistrad is a return to Ravnica (which I'm not sold on), and people assume Shocks would be back then.
I think you're right about the long-term price on the lands, and if we did tweak the formula then it basically tells us to sell now. Sell now, sell next week, sell in a few months – the money is going to be there and not be substantially different if you bought in early enough.

Two things you should think about:

1) The fetchlands are rotating out this September, which increases the likelihood that a new set of really hot lands are printed/reprinted in Innistrad. Shocks? Maybe.

2) Polluted Delta, the most expensive Fetch, is $26 or so on BLP. I can't see Fountain sustaining a price higher than that at any point. Sure, it could spike higher, but I don't think $28 eBay/actual value is possible for long.

With both of those on the table, I would re-tweak the formula slightly and say that selling now is fine. But keeping is fine too – there will most likely be at least one more point when you'll be able to get even more.

The one thing I think you are ignoring in this analysis is the effect of having the shocklands legal in Standard again. While we don't know yet how big the Modern player base is going to be, we do know that Standard is going to be many times the size. The moment shocklands become legal in Standard, the price should rise, because there won't have been enough packs opened to meet demand.

Another key variable is which set they are reprinted in. If it's M13, then prices will not rise as much, because all the lands will be available in a "large set". If it is in a block, then we may see the lands distributed throughout the sets, as was done in Ravnica. In this case, we would see a repeat of the "small set" effect, where some of the lands are scarcer than others. Thus, depending on how the lands are distributed, some of the lands could see higher price increases. Following this logic, we should also consider the likelihood that not all of the shocklands would be reprinted. Wizards' typical pattern is that they only release dual lands in allied color pairs, or (less frequently) enemy color pairs. If the shocklands are reprinted in M13, it will probably only be the allied color ones, leaving the enemy-colored ones to continue to rise in price.

I'm interested to get your (and others) reactions to these scenarios but taken together, they increase the upside potential substantially. For this reason, I would recommend keeping a "buy" recommendation on the shocklands, and taking advantage of any dips in price to add to your inventory, with particular weighting toward the enemy-colored duals.