I want to build a powerful server.
How expensive is a motherboard in which I can have 40GB RAM?
For more than 10 DDR3 slots, the main options are either 12 or 18. Of course a motherboard with this memory capacity won't come all that cheap, it also depends on where you live as prices will vary. For a general idea, here is newegg's list of boards with 12 memory slots, sorted from lowest price to highest (prices in USD).