I don't believe anyone mentioned the "easy pay" life membership plan. It's $25 per quarter for 7.5 years. That's the way I became a life member, and it was financially painless. I find being an NRA member to be a worthwhile membership with no real drawbacks. Some have criticised it for constantly begging for money. As far as I know, you can't join any organization without that happening. With the NRA you at least get something for the money you have spent.
I just copied this from the NRA FAQ website:
Q: I would like to become a Life Member of the NRA. Are there any payment plans available?
A: To fit the needs of most members, we offer the Easy Pay Life plan. Under this plan, members make minimum quarterly payments of $25.00 until the full amount of $750.00 is paid. If you would like to get started or would like some more information, please contact NRA Life Member Services at 800-672-3888.