$0; use StartSSL. They're in the trusted root CAs for Chrome and Firefox which are the only browsers that support SPDY so far (I think they're trusted in all major browsers, but I don't spend my day examining IE6's list of trusted CAs)
The actual cost is in requiring a dedicated IP per SSL site, which many hosts charge monthly for. Unless SNI is built into the spec (I browsed around briefly and didn't see a mention, other than a request for it being a requirement in a forum mid-2011), anyone intending to host multiple HTTP2.0/SPDY sites on the same box will need to buy a dedicated IP per domain. It's also something of a pain to add SNI support to an existing host, but so will adding the SPDY support.
The actual cost is in requiring a dedicated IP per SSL site, which many hosts charge monthly for. Unless SNI is built into the spec (I browsed around briefly and didn't see a mention, other than a request for it being a requirement in a forum mid-2011), anyone intending to host multiple HTTP2.0/SPDY sites on the same box will need to buy a dedicated IP per domain. It's also something of a pain to add SNI support to an existing host, but so will adding the SPDY support.