That’s a difficult question to answer because email marketing is typically about retaining existing customers or moving new prospects through the sales process while telemarketing is typically used for generating new prospects.
Lead generation using email marketing usually means one of two things:

1) Email prospecting: Sponsored advertising piggybacking on the broadcasts of other organizations, or *gasp* sending unsolicited mail

2) Marketing automation: Using automated email marketing to engage raw prospects generated through other means until enough information is gathered to mark certain contacts as qualified leads, at which point they are ready to hand off to the sales team

Both telemarketing and email prospecting for raw lead generation are usually quite inefficient and therefore have a high cost per qualified lead. By engaging prospects at the peak moment of their interest – through search – and then engaging with them through forms and landing pages followed up automatically with an email engagement campaign (all elements of marketing automation), you can generate very high quality leads very efficiently and cost effectively.

When measuring the cost per lead from various sources it’s important to also assess the quality of the leads. You can measure quality by scoring fit, counting the opportunities generated, and/or totaling the sales generated. Leads cover the full gamut of quality and timing so if you simply measure cost per lead without considering quality you can easily be lead to believe that cheaper poor quality leads are a better deal than well timed, highly qualified leads that cost a little more.