I just finished my mass mail for 160 firms and it cost me $350 (and I didn't do the big envelopes, which I sorta regret). I e-mailed another 150 and it was free.The way I picked it was I made snail mail my default method. But, if all the site had was "Send Resumes to Denny Crane, Hiring Partner," then I e-mailed.One thing I will say is that when you see a high-quality envelope with high quality paper and "Best" printing inside of it, it really does look good and seem like you took a minute on the firm. If you can add to that a couple lines that individualize the firm, I think it's meaningful.
Some non-nalp firms that do not have SA programs still hire hourly-paid law clerks. While a an SA program is undoubtedly better, that's something else to consider. You might have to contact firms about this directly b/c these hourly positions are often not advertised. Also, be aware that some non-nalp firms have SA programs that are essentially law clerk positions in disguise.