The last time I did the "back of the envelope" calculations (about two weeks ago), it would cost about $12,000,000 to reach 50%. That's the cost to design, contract, receive, and hookup fully-custom ASICs to build a hashmonster.  I believe it was 200 blades, each with 24 ASICs. Each ASIC would have 32 2xSHA256 cores running at 350Mhz, each with the hashes fully-implemented as custom gates, pipelined. (So while we're doing round 3 of the 3rd nonce, we're doing round 2 of the 2nd, and so on.) Each core, once its pipeline was full, could churn out one doublehash every 8 clock cycles. The total hashing power: 6.7THash/s. (That's from memory.)

However, this would only give you half if it was already half. Which it might be, for all we know.