Cheapest place to get bulk chocolate?
In the world of high-end chocolate, Valrhona isn't even the most expensive - in fact, it's practically entry level.  Amedei Signature retails at a whopping $40/lb in "bulk" (1 kg) quantities.

I don't think I've ever seen Valhrona in bulk quantities and/or at a significantly discounted price.  On the other hand, there are similar-quality brands that are way more affordably-priced.  On chocosphere you can buy 22 lbs of Callebaut callets for just over $6 per pound.  Or you can get one of Felchlin's Grand Cru chocolates for about $10 per pound.  You can even buy Guittard there for what appears to be $6.60/lb.  But you'll notice that they even though they also carry Valrhona, they don't have it in bulk.

I also see Candy Warehouse selling the Guittard wafers for, apparently, $3.10/lb if you buy the 25 lb bag.  Chocolate Man sells the Felchlin Maracaibo chocolate (not Grand Cru) for a measly $7.80/lb.  They've got a variety of other well-known brands as well, although their selection is limited, they don't have a lot of the crazy high-end stuff.

Chocovic chocolate also sells at around the $10 range at most stores.  If you can find it, they distribute 10 kg and 25 kg bags (that's 22 lbs and 55 lbs).  I'm not sure where, but if you go to their official site they explicitly mention that they make that size.

You should consider that the exorbitant prices may not be a result of where you're buying from, but of what you're buying.  Valrhona is just expensive.  You may say that is because it's better than all the others, and you may be right (although some would disagree - it's more than a little subjective) - but that is the price you pay for "premium" products.  They can get away with charging $13-$15/lb, so they do.

Do yourself a favour - try some of the lesser-known brands.  A lot of them really do make fantastic chocolate, they're just not as famous as Valrhona.