Some time ago Eran Tromer gave an estimate of single-digit millions of dollars for a device that could factor a 1024 bit key in a year. I can't quite tell what you're suggesting about the useful lifespan of a 1024 bit key, but I feel like Tromer's opinion represents a growing consensus.

You might also think in terms of the security level that a 1024, 2048, 4096 &c key gets you. It isn't 1024 bits for a 1024 bit RSA key!