Select membership is 500$ a year.
Premier membership is 3500$ a year.
You can read all about it at
http://developer.apple.com ...
You'll get access to the newest builds of Apple's software, so you can develop for new trends in Apple's directions early on. You'll also get mailings containing the Seed CDs, so you don't have to download every build of Jaguar you'll get access to but can wait for the mail to arrive. (snail mail...)
If you're an 'advanced user' - as many are, I believe - and want to beta test only (without paying), you can get the betas through illegal (!) means. Often a developer (or a friend of a friend of a developer) will make toast images of developer seed software and upload them to a server of the Gnutella (Limewire), Hotline or Carracho network.
500$ for the Select ADC membership is not too much for a software developer who needs access to the kind of information and software seeds that Apple kindly provides. You'll also get the final versions of Apple's software through the Developer Seed program. Depending on how many products Apple hands out this way, you may even go cheaper that way than pirating the seed software and buying the final products.
Always keep in mind that Apple is the hardware and software company that makes the friendly computers and the operating system (and maybe even the high end applications) you are using to make money with. It's the case with me, and I'm always eager to pay the cheapo prices Apple charges for the great new releases of their software, which made all of my Macs up to now far greater machines than they were when I bought them. Thus I do pay for their software. It's a win-win situation.