hrana,
I don't know how many people will go to a shop just to get an aluminum block into their car. First of all, I think the shop will have to fabricate the block themselves, off the spec of the current block. Just the machine and tooling (minimum $75/hr)labor charge itself will cause you to abandon this idea. You could, however, get lighter engine components such as Titanium pistons/valves/crank, graphite/carbon fiber rings, and composite springs. These will cost you alot of money (>$10,000), but still cheaper than having a custom build block. And if you really want to save more weight, go with carbon fiber hood, carbon fiber tranny, and magnesium rims (watch out for oxidation). I am just kidding on the last few items.
Daniel